Intermediates for fluorinated cation exchange membrane

ABSTRACT

A novel fluorinated cation exchange membrane containing carboxylic acid groups and sulfonic acid groups, both in the form of a specific pendant structure, the carboxylic acid groups being at least 20% on one surface of the membrane and gradually decreasing toward the innerside of the membrane, which membrane is useful in electrolysis of an aqueous alkali metal halide solution with advantageously stable performance for a long term under more severe operational conditions than those conventionally used. The membrane can be prepared from a novel copolymer of a fluorinated olefin with a novel sulfur containing fluorinated vinylether of the formula: ##STR1## wherein k is 0 or 1, l is an integer of 3 to 5, Z is --S-- or --SO 2  -- and R is C 1  -C 10  alkyl, an aryl, C 1  -C 10  perfluoroalkyl.

This application is a divisional of copending application Ser. No.299,164, filed on Sept. 3, 1981, U.S. Pat. No. 4,510,328, which, inturn, is a divisional of application Ser. No. 152,847 filed on May 23,1980, now U.S. Pat. No. 4,329,434.

This invention relates to a novel fluorinated cation exchange membranehaving both carboxylic acid groups and sulfonic acid groups,intermediates and starting materials for production thereof and also toprocesses for producing such materials. This invention also concerns anovel fluorinated cation exchange membrane having sulfonic acid groupswith a high ion-exchange capacity and being provided with physicallyhigh strength.

The cation exchange membrane according to the present invention can beused in electrolysis of an aqueous alkali metal halide solution undermore severe conditions than those conventionally used while maintainingexcellent performance stably for a long time.

In the chlor-alkali industry, wherein caustic soda and chlorine areproduced by electrolysis of sodium chloride, the ion-exchange membraneprocess has recently attracted great attention, because it is moreadvantageous in various aspects such as prevention of environmentalpollution and economical saving of energy than the mercury process andthe diaphragm process of the prior art and also because it can producecaustic soda having substantially the same quality as that produced bythe mercury process.

The greatest factor which controls the economy of the ion-exchangemembrane process is the characteristic of the cation exchange membraneemployed. It is necessary for the cation exchange membrane to satisfythe requirements as set forth below.

(1) To have a high current efficiency and a low electric resistance. Inorder to have a high current efficiency, the membrane is required tohave a sufficiently high ion-exchange capacity and low water content,thus giving a high concentration of fixed ions in the membrane. On theother hand, to the effect of lower electric resistance, a higher watercontent is rather more advantageous. Since the water content will varydepending on the types of ion-exchange groups, the ion-exchange capacityand the concentration of external liquids, it is necessary to select theoptimum combination of these factors.

(2) To be resistant to chlorine and alkali at higher temperatures for along time. A cation exchange membrane comprising a fluorinated polymercan be sufficiently resistant generally under the aforesaid atmosphere,but some membranes may be insufficient in chemical stability dependingon the ion-exchange groups contained therein. Accordingly, it isimportant to select suitable ion-exchange groups.

(3) To be durable for a long time under various stresses working inhighly concentrated alkali under the conditions of high temperature andhigh current density such as a stress of swelling and shrinking, astress accompanied by vigorous migration of substances to effectpeel-off of layers and a stress by vibration of the membrane accompaniedwith gas generation to cause bending cracks. Generally speaking, thephysical strength of the membrane is different depending on the physicalstructure of the membrane, the polymeric composition, the ion-exchangecapacity and the types of ion-exchange groups. Therefore, it isnecessary to realize the optimum selection of these factors.

(4) To be easily produced and low in cost.

In the prior art, there have been proposed several fluorinated cationexchange membranes for use in electrolysis of an aqueous alkali metalhalide solution. For example, there is known a fluorinated cationexchange membrane having pendant sulfonic acid groups prepared byhydrolysis of a copolymer comprising tetrafluoroethylene andperfluoro-3,6-dioxa-4-methyl-7-octene sulfonylfluoride.

Such a well-known fluorinated cation exchange membrane containing onlysulfonic acid groups, however, is liable to permit permeation ofhydroxyl ions migrated and diffused from the cathode compartmenttherethrough due to the high water content afforded by the sulfonic acidgroups. For this reason, such a membrane is disadvantageously low incurrent efficiency. In particular, when electrolysis is conducted, forexample, by recovering a highly concentrated caustic soda solution of20% or higher, the current efficiency is extremely low to a greateconomical disadvantage as compared with electrolysis by the mercuryprocess or the diaphragm process of the prior art.

For improvement of such a drawback of low current efficiency, theion-exchange capacity of sulfonic acid groups may be lowered to, forexample, 0.7 milliequivalent or lower per one gram of the H-form dryresin, whereby the water content in the membrane can be decreased tomake the fixed ion concentration in the membrane higher than themembrane with higher ion-exchange capacity. As the result, the currentefficiency at the time of electrolysis can slightly be prevented frombeing lowered. For example, when electrolysis of sodium chloride isperformed while recovering caustic soda of 20% concentration, thecurrent efficiency can be improved to about 80%. However, improvement ofcurrent efficiency by reduction in ion-exchange capacity of the membranewill cause a noticeable increase in the electric resistance of themembrane, whereby no economical electrolysis is possible. Moreover, atany higher value of the electric resistance of the membrane, it is verydifficult to prepare a commercially applicable sulfonic acid typefluorinated cation exchange membrane improved in current efficiency toabout 90%.

On the other hand, British Pat. No. 1,497,748 and Japanese publishedunexamined patent application No. 126398/1976 disclose fluorinatedcation exchange membranes having carboxylic acid groups as ion-exchangegroups. In these membranes, the fixed ion concentration can be madehigher due to the lower water content of carboxylic acid groups andtherefore the current efficiency can be improved to 90% or higher. Suchmembranes are also chemically stable under the conditions conventionallyused.

When compared at the same level of the ion-exchange capacity, however,the membrane having carboxylic acid groups is higher in electricresistance than the membrane having sulfonic acid groups. Particularly,when used at a high current density, the power unit may be undesirablyvery high. Moreover, perhaps due to lower water content throughout themembrane, the membrane is prone to shrink when used for a long time in ahighly concentrated alkali under severe conditions until it is hardenedso as to be brittle, resulting in layer peel-off or crack formation,whereby current efficiency may disadvantageously be lowered.

For improvement of such drawbacks of the membrane having only carboxylicacid groups, there is also known a cation exchange membrane prepared bybonding films of a fluorinated polymer having carboxylic acid groups orgroups convertible to carboxylic acid groups (hereinafter referred to asprecursors) and a fluorinated polymer having sulfonic acid groups orprecursors thereof or by molding a blend of said polymers into a film,followed by hydrolysis, as disclosed by Japanese published unexaminedpatent application Nos. 36589/1977 and 132089/1978 and U.S. Pat. No.4,176,215. However, these polymers are poorly compatible with each otherand it is difficult to effect complete bonding or blending. When usedunder severe conditions, such a membrane is liable to suffer frompeel-off or formation of cracks and thereby to cause troubles. Theblended product is also entirely insufficient from the standpoint ofcomplete utilization of higher current efficiency of carboxylic acidgroups and lower electric resistance of sulfonic acid groups. It merelyexhibits the intermediate characteristic of both properties.

The aforesaid Japanese published unexamined patent applications andanother Japanese published unexamined patent application No. 23192/1977also disclose a cation exchange membrane prepared by ternarycopolymerization of a vinyl monomer having carboxylic acid groups orprecursors thereof, a vinyl monomer having sulfonic acid groups orprecursors thereof and a fluorinated olefin, followed by fabricationinto a film and hydrolysis. Such a membrane also merely shows theintermediate characteristic.

On the other hand, there are disclosed cation exchange membranesprepared by forming carboxylic acid groups by chemical treatment on onesurface of fluorinated cation exchange membranes having sulfonic acidgroups, as disclosed by U.S. Pat. No. 4,151,053, Japanese publishedunexamined patent application Nos. 104583/1978, 116287/1978 and6887/1979. These membranes, due to the presence of carboxylic acidgroups, will effectively inhibit migration and diffusion of hydroxylions to exhibit higher current efficiency. Also, since the carboxylicacid groups are present in the thin layer on the cathode side andsulfonic acid groups with higher water content in the residual part ofthe membrane, the electric resistance of the membrane is low. Thus,these membranes are very excellent from the standpoint of powerconsumption. However, all of these membranes, while they are stably usedwith good performance under conventional conditions for a commerciallysatisfactory term, will suffer under severe conditions of furtherincreased high current density and high temperature from swelling likesplotch or formation of water bubbles, peel-off of the carboxylic acidlayer from the sulfonic acid layer or formation of cracks in thecarboxylic acid layer, thereby causing a decrease in current efficiency,as shown in the comparative examples.

It has not yet been clarified why such pehnomena are caused. Presumably,the polymeric structure of the fluorinated cation exchange membranehaving sulfonic acid groups or derivatives thereof may be one of thefactors for such phenomena. That is, these membranes are prepared bychemical treatment of a copolymer of a fluorinated olefin with a sulfurcontaining fluorinated vinylether as represented by the followingformula formed in the shape of a membrane or a hydrolyzed productthereof having sulfonic acid groups: ##STR2## wherein n' is an integerof 0 to 2.

Among said monomers, the monomer of n'=0 will cause the cyclizationreaction as shown by the reaction scheme (1) below in the vinylizationstep as disclosed by Japanese published examined patent application No.2083/1972. ##STR3## For converting the cyclic sulfone to CF₂ ═CFOCF₂ CF₂SO₂ F, a number of reaction steps are required to be performed andtherefore it is very difficult to produce said monomer in commercialapplication. Moreover, depending on the conditions, such cyclizationwill also occur at the time of polymerization and may lower theproperties of the resultant polymer.

For this reason, in commercial application, the monomer of n'=1 isconventionally used. With such a monomer, there is the drawback that theion-exchange capacity of the resultant sulfonic acid type membrane andthe membrane having formed carboxylic acid groups by chemical treatmenton the surface of the sulfonic acid type membrane can limitedly beincreased, as disclosed by the aforesaid Japanese published unexaminedpatent applications. Furthermore, perhaps due to the presence of thependant groups: ##STR4## no physically tough membrane can be obtainedunless the copolymerization ratio of a fluorinated olefin to the sulfurcontaining fluorinated vinyl ether is increased to about 6 or more. Itis also expected that use of such a monomer may be one of the factorscausing peel-off or cracks of the carboxylic acid layer formed whenusing the membrane having carboxylic acid groups and sulfonic acidgroups as mentioned above under more severe conditions thanconventionally used. The above drawbacks are further multiplied when themonomer of n'=2 a having larger molecular weight is used.

A copolymer of a fluorinated vinyl monomer having no ether linkage suchas trifluorovinyl sulfonyl fluoride with tetrafluoroethylene, asdisclosed by U.S. Pat. No. 3,624,053, is deficient in fabricability intoa membrane.

Japanese published unexamined patent application Nos. 28588/1977,23192/1977 and 36589/1977 disclose fluorinated cation exchange membranesprepared from copolymers of fluorinated olefins with fluorinated vinylcompounds represented by the formula:

    CF.sub.2 ═CX.sup.1 (OCF.sub.2 CFX.sup.2).sub.a O.sub.b (CFX.sup.3).sub.c SO.sub.2 X.sup.4

wherein X¹ is F or CF₃, X² and X³ are F or C₁ -C₁₀ perfluoroalkyl, X⁴ isF, OH, OQ¹, OM and NQ² Q³ (Q¹ is C₁ -C₁₀ alkyl, and Q² and Q³ are H orone of Q¹, and M is an alkali metal or quaternary ammonium), a is aninteger of 0 to 3, b an integer of 0 or 1 and c an integer of 0 to 12.However, these prior publications refer to no typical example of aprocess for preparation of said fluorinated vinyl compounds. Nothing istaught about precursors of said compounds. Moreover, as clearly seenfrom the description in the specifications of said Japanese publishedunexamined patent applications, there is only disclosure of thecompounds, copolymers and membranes derived therefrom in the Examplesand preferred typical examples which are those conventionally known ofthe formula: ##STR5## wherein a is the same as defined above, namely thegroup of compounds wherein c is 2, although preferred embodiments arementioned to be those wherein X¹ ═F, X² ═CF₃, X³ ═F or CF₃, X⁴ ═F. a=0to 1, b=1 and c=1 to 3.

In the field of ion-exchange membranes, it is strongly desired todevelop a membrane which exhibits high current efficiency and lowelectric resistance under more severe conditions, has a longer life andcan be produced at low cost. The present inventors have made efforts todevelop such a membrane and consequently found that the above object canbe attained by use of a novel fluorinated vinyl ether compound which isderived from starting materials having specific structure. The presentinvention has been accomplished based on such a finding.

The first object of the present invention is to provide a fluorinatedcarboxylic acid or its derivative represented by the formula:

    X(CF.sub.2).sub.n Y

wherein X stands for --SR¹ or --SO₂ R² (R¹ is an alkyl having 1 to 10carbon atoms, an aryl, a perfluoroalkyl having 1 to 10 carbon atoms orchlorine; and R² is R¹ or --OM, M indicating hydrogen, a metal orammonium group); Y stands for --COY¹ or --CN, Y¹ being halogen,hydrogen, --NH₂, --OM (M is the same as defined above), or --OR³ (R³ isan alkyl having 1 to 10 carbon atoms or an aryl); and n stands for aninteger of 2 to 4, and a process for producing the same.

In the prior art, as a fluorinated compound having in combinationcarboxylic acid derivative groups and sulfonic acid groups or groupsconvertible thereto in the same molecule such as said fluorinatedcarboxylic acid derivative groups, there is known only the compound FSO₂CF₂ COF or the compound ##STR6## as disclosed by U.S. Pat. No.3,301,893. There is no suggestion about a compound comprising afluorinated alkylene group having 2 to 4 carbon atoms --CF₂ --_(n)between the carboxylic acid derivative groups and sulfonic acid groupsor the groups convertible thereto such as the compound according to thepresent invention.

The fluorinated carboxylic acid derivative according to the presentinvention can be prepared by converting the compound obtained by aprocess comprising the following step (A), (B) or (C) according to thereaction scheme (3), (4), (5) or (6), optionally in combination withvarious reactions such as acid treatment, hydrolysis treatment orhalogenation treatment, into a carboxylic acid derivative and sulfonicacid derivative:

(A) A method comprising the step to react tetrafluoroethylene with acarbonic acid ester having 3 to 20 carbon atoms in the presence of amercaptide represented by the formula R'SM¹ (R' is an alkyl having 1 to10 carbon atoms, an aryl or a perfluoroalkyl having 1 to 10 carbonatoms; M¹ is an alkali metal, ammonium group or a primary to quaternaryalkylammonium group): ##STR7## (wherein R⁴ and R⁵ represent alkyl oraryl, and M¹ is the same as defined above);

(B) A method comprising the step to react tetrafluoroethylene with acompound of the formula: A'₂ SO₂ (A' is a halogen or an alkoxy having 1to 5 carbon atoms) in the presence of an alkali cyanide:

    NaCN+CF.sub.2 ═CF.sub.2 +A'.sub.2 SO.sub.2 →NCCF.sub.2 CF.sub.2 SO.sub.2 A'                                               (4)

(wherein A' is the same as defined above);

(C) A method comprising the step to react tetrafluoroethylene with acompound of the formula: Z'SO₂ F or Z'₃ CSO₂ F (Z' is a halogen exceptfor F) in the presence of a free radical initiator: ##STR8##

In the fluorinated carboxylic acid derivative of the present inventionX(CF₂)_(n) Y (X, Y and n are the same as defined above), n maypreferably be 2 when considering ease of preparation and the molecularweight of the fluorinated vinyl monomer prepared from said derivative.The group X may preferably be --SR¹ or --SO₂ R¹, especially X=--SR¹being preferred. As the group R¹, an alkyl having 1 to 10 carbon atomsor an aryl, especially an alkyl having 1 to 10 carbon atoms ispreferred. Among them, an alkyl having 1 to 5 carbon atoms is mostpreferred. A compound wherein Y is --COF is also desirable from thestandpoint of usefulness as a starting material for the synthesis of afluorinated vinyl compound. When Y is another carboxylic acidderivative, such a compound may be converted to a compound having thegroup Y=--COF.

Each of the methods (A), (B) and (C) is hereinafter described in furtherdetail.

I. Method (A)

Examples of mercaptide to be used in the method (A) are derivatives ofmethyl mercaptan, ethyl mercaptan, propyl mercaptan, butyl mercaptan,amyl mercaptan, hexyl mercaptan, phenyl mercaptan, benzyl mercaptan,toluyl mercaptan, perfluoromethyl mercaptan, perfluoroethyl mercaptan,perfluoropropyl mercaptan, etc. in the form of sodium salts, potassiumsalts, cesium salts, ammonium salts, and primary to quaternaryalkylammonium salts, preferably an alkyl mercaptan, especially having 1to 5 carbon atoms, namely methyl-, ethyl-, propyl-, butyl andamyl-mercaptan in the form of sodium salts or potassium salts.

The carbonic acid ester may be exemplified by dimethyl-, diethyl-,dipropyl-, dibutyl-, diphenyl-, or methylethyl-carbonate. Preferably,dimethyl carbonate and diethyl carbonate may be used.

The mercaptide and the carbonic acid ester are usually mixed in an inertmedium. But no inert medium is necessarily required when said ester isliquid under the reaction conditions. Typical examples of a suitableinert medium are diethyl ether, tetrahydrofuran, dioxane, ethyleneglycol dimethyl ether, diethylene glycol dimethyl ether, benzene andcyclohexane, having no active hydrogen and being capable of dissolvingthe carbonic acid ester.

The carbonic acid ester is used in an amount of 0.1 to 10 equivalents,preferably 0.5 to 5 equivalents, of the mercaptide.

Tetrafluoroethylene is usually employed in the gaseous state and may befed into the reaction system under any desired pressure, irrespective ofwhether it may be pressurized, normal or reduced. Tetrafluoroethylenemay be added in an amount of 0.1 to 5 equivalents, preferably 0.4 to 3equivalents of the mercaptide.

The reaction is carried out usually at not higher than 100° C.,preferably in the range from 80° to 0° C., until the pressure oftetrafluoroethylene is substantially constant under the reactionconditions employed. Formation of ketone leads to substantial decreasein the reaction yield based on the mercaptide. For this reason, it ispreferred to use a lower temperature in order to suppress formation ofthe ketone in the reaction scheme (3). The reaction is carried out undersubstantially anhydrous conditions.

After completion of the reaction, the reaction system is made acidic byadding an acid. In this case, such a mineral acid as hydrochloric acid,sulfuric acid or phosphoric acid is usually used, sulfuric acid beingpreferred. The amount of a mineral acid should be at least equivalent tothe mercaptide initially employed.

In the above reaction procedure, there may also be used in place of thecarbonic acid ester a N,N-dialkyl formamide having 3 to 7 carbon atoms,whereby a fluorinated aldehyde is obtained. Alternatively, in somecases, there may also be employed carbonic acid gas in place of thecarbonic acid ester.

Isolation of ester, ketone or aldehyde which is the fluorinatedcarboxylic acid derivative may be performed by a conventional techniqueof separation such as phase separation, distillation or others. Saidfluorinated carboxylic acid derivative of ester, ketone or aldehyde maybe converted to various carboxylic acid derivatives according tosuitable organic reaction procedures. For example, ester and ketone maybe hydrolyzed with an alkali to give a carboxylic acid salt, whichcarboxylic acid salt may in turn be treated with a mineral acid to givea carboxylic acid. Further, the above carboxylic acid or salt thereofmay be reacted with a chlorinating agent such as phosphoruspentachloride, thionyl chloride, etc. to obtain an acid chloride, oralternatively with sulfur tetrafluoride to obtain an acid fluoride.Also, according to the well known reaction to treat an acid chloridewith sodium fluoride or potassium fluoride, an acid fluoride can beprepared. An acid fluoride is most useful from the standpoint of thestarting material for synthesis of a fluorinated vinyl compoundaccording to the reaction scheme (7) as shown below; ##STR9## wherein nand X are the same as defined above, and l' is 1 or 2.

In the above fluorinated carboxylic acid derivative, the sulfide grouppresent on the terminal end opposite to that of carboxylic acidderivative group may also be converted to various derivatives accordingto suitable organic reaction procedures. For example, it may beconverted by treatment with chlorine to the sulphenyl chloride group orsulfonyl chloride group, or by oxidation treatment to the sulfone group.Further, these groups may be subjected to hydrolysis treatment with analkali to be converted to sulfonic acid group salts, which may betreated with phosphorus pentachloride to be converted to sulfonylchloride groups. Conversion to such various derivative groups does notinterfere with the reaction according to the scheme (7), insofar as suchgroups have no active hvdrogen.

II. Method (B)

The alkali metal cyanide to be used in the method (B) may includecyanides of lithium, sodium, potassium, cesium, etc. Among them,cyanides of sodium and potassium may preferably be used.

Examples of the compound of the formula A'₂ SO₂ are sulfuryl fluoride,sulfuryl chloride, sulfuryl bromide, sulfuryl chlorofluoride, sulfurylbromofluoride, dimethyl sulfate, diethyl sulfate, dibutyl sulfate,diamyl sulfate, and the like. In some cases, there may also be usedsulfur dioxide.

The alkali metal cyanide is used usually as a dispersion in an inertmedium. When the compound A'₂ SO₂ (A' is the same as defined above) is aliquid under the reaction conditions, no such inert medium isnecessarily required to be used.

As a suitable inert medium, there may be mentioned solvents having noactive hydrogen such as diethyl ether, tetrahydrofuran, dioxane,ethylene glycol dimethyl ether, diethylene glycol dimethyl ether,benzene, cyclohexane, etc. Said inert medium may desirably be capable ofdissolving A'₂ SO₂.

The A'₂ SO₂ is used in an amount of 0.1 to 10 equivalents, preferably0.5 to 5 equivalents of the alkali metal cyanide.

Depending on the A'₂ SO₂ employed and the properties thereof, A'₂ SO₂ ispreviously charged in the reaction system to be mixed with the alkalimetal cyanide, or fed into the reaction system simultaneously withtetrafluoroethylene, or fed into the reaction system previously mixedwith tetrafluoroethylene.

Tetrafluoroethylene is used usually under the gaseous state and may befed into the reaction system under any desired pressure, whether it maybe pressurized, reduced or normal.

Tetrafluoroethylene is added in an amount of 0.1 to 5 equivalents,preferably 0.4 to 3 equivalents of the alkali metal cyanide.

The reaction is carried out at not higher than 250° C., preferably atnot higher than 100° C., until the pressure of tetrafluoroethylene issubstantially constant under the reaction conditions employed. Thereaction is conducted under substantially anhydrous conditions.

Separation of fluorinated nitrile may be performed according to suchprocedures as phase separation or distillation. Similarly as describedin the method (A), said fluorinated nitrile may be converted to variouscarboxylic acid derivatives or sulfonic acid derivatives according tosuitable organic reaction procedures, whereby it is most preferred thatY should be --COF.

III. Method (C)

The compound represented by the formula Z'SO₂ F or Z'₃ CSO₂ F (Z' is thesame as defined above) to be used in the method (C) may be exemplifiedby sulfuryl chlorofluoride, sulfuryl bromofluoride, trichloromethanesulfonylfluoride, tribromomethane sulfonylfluoride, and the like. Amongthem, sulfuryl chlorofluoride and trichloromethane sulfonylfluoride arepreferred.

As the free radical initiator, there may be employed most of thoseconventionally used in the field of organic chemical reactions. Forexample, it is possible to use organic peroxides such as benzoylperoxide, di-t-butyl peroxide, perfluoroacetyl peroxide, di-t-amylperoxide, etc. and azo-bis type compounds such asazobisisobutyronitrile, azobisisovaleronitrile, azobisnitrile, etc.

In the present invention, instead of permitting the free radicalinitiator to be present in the reaction, ultra-violet irradiation may beemployed. Alternatively, it is also possible to effect irradiation ofultra-violet rays in the presence of a free-radical initiator.

Use of a solvent is not particularly limited, but there may be employedany solvent which is stable chemically to the free radical initiator orultra-violet rays. Particularly, 1,1,2-trichloro-1,2,2-trifluoroethaneand cyclohexane may preferably be used.

Tetrafluoroethylene is used in at least a stoichiometric amount relativeto Z'SO₂ F or Z'₃ CSO₂ F.

The amount of the free radical initiator used is in the range from0.001% to 10% based on Z'SO₂ F or Z'₃ CSO₂ F.

The reaction temperature may suitably be determined in view of thehalf-life period of the free radical initiator or other factors, usuallyranging from -10° C. to 250° C., preferably from 0° C. to 150° C.

After completion of the reaction, the intermediates formed according tothe reaction scheme (5) or (6) are isolated by phase separation ordistillation from the reaction mixture, if desired. Said intermediatesmay be subjected to acid treatment using a mineral acid such as conc.sulfuric acid, sulfuric anhydride or fuming nitric acid to be convertedto HOOC(CF₂)₃ SO₂ F or HOOC(CF₂)₄ SO₂ F.

The above carboxylic acid may be isolated from the reaction mixture byan isolation procedure such as extraction, phase separation ordistillation. Similarly as described in the method (A), said carboxylicacid may be converted to various carboxylic acid derivatives accordingto suitable organic chemical reaction procedures. It is particularlypreferred that Y should be --COF. Among various sulfonic acidderivatives, sulfonylfluoride groups can be converted to sulfone andsulfide groups.

According to another preparation method, it is also possible to carryout reaction between a disulfide and tetrafluoroethylene in the presenceof a free radical initiator to give an intermediate having sulfidegroups at both terminal ends of the molecule, which intermediate is thensubjected to chlorine treatment to provide a compound having a sulfidegroup at one terminal end and a sulfonyl group at the other terminalend. By treatment of said compound with hydroiodic acid, there may alsobe prepared a compound having the sulfide group and the carboxylic acidgroup according to the present invention.

Alternatively, a compound having a sulphenylchloride group andsulphenyliodide group may be allowed to react with tetrafluoroethylenein the presence of a free radical initiator, followed by treatment ofthe resultant intermediate with an acid such as conc. sulfuric acid,sulfuric anhydride or fuming nitric acid, to provide the compound of thepresent invention having both a sulfide group and carboxylic acid group.

The compound of the present invention, especially an acid fluoride, isvery useful for synthesis of a fluorinated vinyl ether compound havingterminal groups convertible to sulfonic acid groups as shown in thereaction scheme (7). The above compound is also useful as startingmaterials for production of various materials such as surfactants, fibertreatment agents, lubricants, agricultural chemicals, etc.

The fluorinated carboxylic acid derivative of the present invention canalso very advantageously be produced, since no dangerous reaction isused such as the addition reaction between tetrafluoroethylene and SO₃which will occur in the production of FSO₂ CF₂ COF and also no toxiccompound such as a cyclic sultone intermediate is involved.

The second object of the present invention is to provide a novelfluorinated acid fluoride represented by the formula: ##STR10## whereinX' is --SR or SO₂ R (R is C₁ -C₁₀ alkyl, C₁ -C₁₀ perfluoroalkyl, aryl orchlorine), n is an integer of 2 to 4, p is an integer of 0 to 50, and aprocess for producing said fluorinated acid fluoride compound whichcomprises reacting a novel compound represented by the formula:

    X'(CF.sub.2).sub.n COF

wherein X' and n are the same as defined above, withhexafluoropropyleneoxide, in the presence of a fluoride ion.

As a fluorinated compound having in combination an acid fluoride groupand a functional group convertible to a sulfonic acid group in the samemolecule such as said fluorinated acid fluoride compound, there is knownin the prior art only a fluorinated acid fluoride of the followingformula: ##STR11## wherein l"=2, q'=0-50, as disclosed by Japanesepublished examined patent application No. 1664/1967. No such compound ofthe present invention wherein l" is 3 to 5 is suggested at all in theprior art.

The fluorinated acid fluoride of the present invention can be producedaccording to the following reaction scheme: ##STR12## wherein X', n andp are the same as defined above.

The reaction between the compound of the formula X'(CF₂)_(n) COF(wherein X' and n are the same as defined above) and hexapropylene oxidemay preferably be conducted in the presence of a fluoride ion ascatalyst. This can easily be done by use of a suitable fluoride,including alkali metal fluorides such as cesium fluoride, potassiumfluoride, etc.; silver fluoride; ammonium fluoride; C₁ -C₄ tetraalkylammonium fluoride such as tetramethyl ammonium fluoride, tetraethylammonium fluoride and tetrabutyl ammonium fluoride; and so on.

The fluoride catalyst is usually used together with an inert liquiddiluent, preferably an organic liquid, which can dissolve at least0.001% of the fluoride selected. The fluoride catalyst may be used in anamount of about 0.01 to about 2 mole equivalent per one mole of thecompound represented by the formula X'(CF₂)_(n) COF wherein X' and n arethe same as defined above. Examples of suitable diluents are polyetherssuch as ethyleneglycol dimethylether, diethyleneglycol dimethylether,tetraethyleneglycol dimethylether, etc. and nitriles such asacetonitrile, propionitrile, etc. The reaction is slightly exothermicand therefore there should be provided a means for dissipating thereaction heat.

The reaction temperature may be in the range from about -50° C. to about200° C., preferably from about -20° C. to about 150° C. The pressure isnot a critical parameter and may either be lower than or not lower thanthe atmospheric pressure. The reaction time may usually be from 10minutes to 100 hours. The applicable molar ratio of hexapropylene oxideto X'(CF₂)_(n) COF is from about 1/20 to about 100/1. When the compound##STR13## has a low p value, for example, when p is 0 or 1, the relativeproportion of X'(CF₂)_(n) COF is increased, and lower pressure andhigher temperature are preferred to be selected. On the other hand, whena product with a high p value is desired to be prepared, it is preferredto increase the relative proportion of hexapropylene oxide and selecthigher pressure and lower temperature.

In the fluorinated acid fluoride of the present invention, ##STR14##wherein X', n and p are the same as defined above, a compound whereinn=2 and also a compound where X'=13 SR are preferred from the standpointof ease of preparation. As the group R, C₁ -C₁₀ alkyl or an aryl,especially C₁ -C₁₀ alkyl is preferred. Among them, C₁ -C₅ alkyl is mostpreferred.

On the other hand, a cation exchange membrane prepared from a copolymerof said fluorinated vinyl ether compound and tetrafluoroethylene maydesirably have an ion-exchange capacity as large as possible. From thisstandpoint, said fluorinated vinyl ether compound may preferably have amolecular weight as small as possible. Accordingly, it is preferred thatthe value of p may be 0 or 1, most preferably 0.

The compound represented by the formula: ##STR15## wherein X', n and pare the same as defined above is useful as an intermediate for thepreparation of a novel fluorinated vinylether compound having functionalgroups convertible to sulfonic acid groups. Said compound is also usefulas a starting material for surfactants, fiber treatment agents,lubricants, agricultural chemicals, etc.

The third object of the present invention is to provide a novelfluorinated vinylether compound represented by the formula: ##STR16##wherein X' is --SR or SO₂ R (R is C₁ -C₁₀ alkyl, an aryl, C₁ -C₁₀perfluoroalkyl or chlorine, n an integer of 2 to 4 and p' an integer of0 to 5, and a process for preparing the same.

As a fluorinated vinylether compound having functional groupsconvertible to sulfonic acid groups such as said fluorinated vinylethercompound, there is known in the prior art only the class of compounds:##STR17## wherein l"=2 and m'=0 to 2. Nothing is suggested in the priorart about the compounds of the present invention wherein l" is 3 to 5.

The fluorinated vinylether compound of the present invention can beprepared according to the following reaction schemes: ##STR18## whereinX', n and p' are the same as defined above, and W is F or OM' (M' is analkali metal).

The fluorinated vinylether compound of the present invention representedby the formula ##STR19## wherein X', n and p' are the same as definedabove, can be prepared by pyrolysis of the compound of the formula:##STR20## wherein X', n, p' and W are the same as defined above,according to the aforesaid scheme (II). In said reaction, it ispreferred to use a compound wherein W=F from the standpoint of ease ofuse in the reaction.

Said reaction can be practiced under substantially anhydrous conditionsunder either pressurized, normal or reduced pressure. Usually, however,the reaction may conveniently be practiced under normal or reducedpressure

There may also be employed a diluent to a dilution degree of 0 to 100depending on the mode of reaction, said diluent being selected frominert gases such as nitrogen, helium, carbon dioxide, argon, etc. orinert non-protonic liquids such as polyethers.

When the terminal group is an acid fluoride group, it is possible anddesirable to carry out the reaction in the presence of a metallic saltor a metal oxide. In this case, there may preferably be used a solidbase which can decompose any corrosive and toxic COF₂ generated such assodium carbonate, potassium carbonate, sodium phosphate, potassiumphosphate, etc.

The reaction temperature may range from 100 to 600° C., preferably from100° to 350° C. If the temperature is too high, side reactions such asdecomposition other than vinylization are liable to occur. At too low atemperature, conversion of the starting material is lowered. Thereaction time may be from 0.1 second to 10 hours, preferably from 10seconds to 3 hours. The reaction temperature and the reaction timeshould suitably be selected to provide optimum conditions, for example,shorter reaction time at higher reaction temperature or longer reactiontime at lower reaction temperature.

In the prior art, it has been deemed commercially difficult to prepareFSO₂ (CF₂)₂ OCF═CF₂ by a process comprising pyrolyzing ##STR21## (m' isthe same as defined above) to form the corresponding fluorinatedvinylether compound ##STR22## because a cyclization reaction will occurwhen m' is 0.

In contrast, according to the present invention, use is made of thefluorinated acid fluoride represented by the formula ##STR23## whereinX', n and p' are the same as defined above. Thus, due to the differencein size of the ring, pyrolysis can be effected while causing no or onlya negligible cyclization reaction. Therefore, it is possible to produceeasily a fluorinated vinylether compound represented by the formula:##STR24## wherein X', n, p' are the same as defined above, even when p'may be 0. Said fluorinated vinylether compound is also free fromcyclization during polymerization, thereby causing no deterioration ofproperties of the resultant polymer.

In the fluorinated vinylether compound of the present invention##STR25## wherein X', n and p' are the same as defined above, it ispreferred from the standpoint of ease of preparation that n is equal to2 and X' equal to --SR. In said group, R may preferably be C₁ -C₁₀ alkylor an aryl, especially C₁ -C₁₀ alkyl, most preferably C₁ -C₅ alkyl.

On the other hand, the cation exchange membrane to be prepared from thecopolymer of said fluorinated vinylether compound andtetrafluoroethylene is desired to have an ion-exchange capacity as largeas possible. From this standpoint, said fluorinated vinylether compoundmay preferably be one wherein p' is equal to 0 or 1, especially p'=0being preferred.

The fluorinated vinylether compound of the present invention can becopolymerized with, for example, tetrafluoroethylene to give afluorinated cation exchange membrane which has the very excellentcharacteristic of sufficiently high ion-exchange capacity whilemaintaining good mechanical strength.

The fluorinated vinylether compound of the present invention may also beuseful as an intermediate for synthesis of various fluorinated compoundshaving functional groups containing a sulfur atom at the terminal end ofthe molecule, for example, surfactants, fiber treating agents,lubricants, etc. It is also possible to prepare a fluorinated elastomercomprising a copolymer of the above fluorinated vinylether compound witha fluorinated olefin using said compound as a constituent orcrosslinking monomer of said elastomer.

The fourth object of the present invention is to provide a novelfluorinated copolymer comprising the following recurring units (A) and(B):

    --CA.sub.1 A.sub.2 --CA.sub.3 A.sub.4 --                   (A)

(A₁ and A₂ are F or H; A₃ is F, Cl or H; A₄ is F, Cl, CF₃, --OR_(F), Hor CH₃, R_(F) being C₁ -C₅ perfluoroalkyl) ##STR26## (k=0 or 1; l is aninteger of 3 to 5; Z is --S-- or --SO₂ --; R is C₁ -C₁₀ alkyl, an aryl,Cl or C₁ -C₁₀ perfluoroalkyl)

and a process for producing the same. In the above copolymer, the ratioof the numbers of recurring unit (A)/(B) is desired to be in the rangefrom 1 to 16.

When the copolymer is required particularly strongly to have resistanceto heat and chemicals, as is required in preparation of a fluorinatedcation exchange membrane for use in electrolysis of an aqueous alkalimetal halide solution, the recurring unit (A) in the above formula maypreferably be: ##STR27## (L is F, Cl, CF₃, --OR_(F) or H, R_(F) beingthe same as defined above). It is particularly preferred that L shouldbe F.

In order to produce membranes or resins having high ion-exchangecapacity and physical toughness, the notation k may preferably be zero.The ratio (A)/(B) is preferred to be in the range from 1.5 to 14, morepreferably from 3 to 11.

From standpoint of ease of preparation of the monomer, the physicalproperties of the resultant polymer and possible greater variety of thepolymer properties, it is also preferred that l should be equal to 3 andR should be C₁ -C₁₀ alkyl or an aryl, C₁ -C₁₀ alkyl being especiallypreferred. When taking also polymerizability and moldability intoconsideration, a monomer wherein Z is --S-- and R is C₁ -C₁₀ alkyl,especially C₁ -C₅ alkyl may preferably be used.

The above copolymer is substantially a random copolymer having amolecular weight generally in the range from 8,000 to 1,000,000, havinga melt index generally in the range from 0.001 g/10 min. to 500 g/10min., as measured by use of an orifice of 2.1 mm in diameter and 8 mm inlength, under a load of 2.16 kg at 250° C.

The above copolymer may conveniently be identified by measurement of theinfrared absorption spectrum (IR) or attenuated total reflection (ATR)of a film of the copolymer, as shown in the Examples.

The composition of the copolymer is estimated by measurement of theion-exchange capacity, elemental analysis or a combination thereof afterconverting all of the sulfur containing terminal groups to ion-exchangegroups such as sulfonic acid groups or carboxylic acid groups.

The structure of the pendant groups contained in the copolymer accordingto the invention can also be identified by measurement of the IR or ATRof the product formed by converting the sulfur containing terminalgroups to ion-exchange groups such as sulfonic acid groups, carboxylicacid groups or sulfinic acid groups and then carrying out the reactionfor elimination of said ion-exchange groups.

The fluorinated copolymer of the present invention can be prepared bycopolymerization of at least one monomer selected from the groupconsisting of the olefins of the formula:

    CA.sub.1 A.sub.2 ═CA.sub.3 A.sub.4

wherein A₁, A₂, A₃ and A₄ are the same as defined above, at least onemonomer selected preferably from the group consisting of the fluorinatedolefins of the formula:

    CF.sub.2 ═CFL

wherein L is F, Cl, CF₃, --OR_(F) or H, R_(F) being C₁ -C₅perfluoroalkyl, and at least one monomer selected from the groupconsisting of sulfur containing fluorinated vinylether compounds of theformula: ##STR28## wherein k, l, Z and R are the same as defined above.

In this case, there may also be copolymerized a minor amount of othervinyl compounds mixed with the above monomers. It is also possible toeffect crosslinking by copolymerization of a divinyl compound such asperfluorobutadiene or perfluorodivinylether or a fluorinated vinylcompound having terminal groups capable of effecting a crosslinkingreaction such as CF₂ I, etc.

The fluorinated olefin to be used in the present invention maypreferably be one containing no hydrogen atom from the standpoint ofheat resistance and chemical resistance of the resultant copolymer.Above all, tetrafluoroethylene is most preferred.

Among the sulfur containing fluorinated vinylether compounds, thosewherein k=0 are preferred for providing membranes with greaterion-exchange capacity and excellent physical toughness. Of course, theremay also be used a minor amount of the compound wherein k=1. The classof compound wherein l=3 is also preferred from the standpoint of ease ofpreparation as well as the physical properties of the resultant polymer.A compound with l=6 or more can only be produced with difficulty and canprovide no membrane having sufficiently high ion-exchange capacity, thusbeing inferior to those with l=3 to 5.

The group R may preferably be C₁ -C₁₀ alkyl or an aryl in view of theease in preparation of the vinyl monomer. Among them, C₁ -C₁₀ alkylgroup is more preferable.

When taking also polymerizability and moldability into consideration, itis especially preferred to use a compound wherein Z is --S-- and R is C₁-C₁₀ alkyl.

Typical examples of the sulfur containing fluorinated vinylethercompounds preferably used in the present invention are as follows:##STR29## wherein k is 0 or 1, preferably 0, R is C₁ -C₁₀ alkyl or anaryl.

As compared with the sulfur containing vinylether compoundconventionally used in the prior art for preparation of fluorinatedcation exchange membranes or fluorinated cation exchange resins havingsulfonic acid groups and/or carboxylic acid groups, the sulfurcontaining fluorinated vinylether compound of the present invention issubstantially free from or remarkably decreased in such cyclizationreaction as previously described in the vinylization step, even whenk=0, due to the difference in the number of members constituting thering. Thus, a compound with k=0 can also easily be produced. Also duringpolymerization, there is no deterioration of the polymer properties dueto a cyclization reaction. Accordingly, vinylether compounds with k=0can principally be used in polymerization to provide a fluorinatedcopolymer containing substantially no or only a minor amount of pendant##STR30## As the result, the content of fluorinated olefin can beincreased at the same level as the ion-exchange capacity of themembranes or resins, whereby there can be obtained membranes or resinshaving higher ion-exchange capacity and also having good physicaltoughness.

The ratio of the olefin and the sulfur containing fluorinated vinylether compound to be copolymerized can be controlled by suitableselection of the ratio of monomers charged and the polymerizationconditions.

The copolymer of the present invention may be prepared according to wellknown polymerization methods used for homopolymerization orcopolymerization of a fluorinated ethylene. The methods for preparationof the copolymer of the present invention may include both a method inwhich polymerization is conducted in a non-aqueous system and a methodin which polymerization is conducted in an aqueous system. Thepolymerization temperature may generally range from 0° to 200° C.,preferably from 20° to 100° C. The pressure may be from 0 to 200 kg/cm²,preferably from 1 to 50 kg/cm². The non-aqueous polymerization mayfrequently be carried out in a fluorinated solvent. Suitable non-aqueoussolvents may include inert 1,1,2-trichloro-1,2,2-trifluoroethane orperfluoro-hydrocarbons, e.g. perfluoromethylcyclohexane,perfluorodimethylcyclobutane, perfluorooctane, perfluorobenzene, etc.

As an aqueous polymerization method for preparation of the copolymer,there may be mentioned an emulsion polymerization method whereinmonomers are brought into contact with an aqueous medium containing afree radical initiator and an emulsifier to provide a slurry of polymerparticles or a suspension polymerization method wherein monomers arebrought into contact with an aqueous medium containing both free radicalinitiator and dispersion stabilizer inert to telomerization to provide adispersion of polymer particles, followed by precipitation of thedispersion. As the free radical initiator to be used in the presentinvention, there are redox catalysts such as ammonium persulfate-sodiumhydrogen sulfite, etc.; organic peroxides such as t-butyl peroxide,benzoyl peroxide, etc.; azo-bis type compounds such asazobisisobutyronitrile, and fluorine radical initiators such as N₂ F₂,etc.

After polymerization, the polymer may be molded into membranes orgranules, if desired. A conventional technique may be used for moldingthe polymer into a thin film or pellets by melting the polymer.

The copolymer of the present invention is useful as a starting materialfor preparation of a fluorinated cation exchange membrane havingsulfonic acid groups and/or carboxylic acid groups. In this case, theabove membrane may, sometimes preferably, be laminated with a membranemade from a copolymer having a greater copolymerization ratio of thesulfur containing fluorinated vinylether compound. As the membrane to belaminated, there may be used a membrane prepared from the copolymer ofthe monomers selected from the group of the above sulfur containingfluorinated vinylether compounds and the groups of fluorinated olefins.Alternatively, there may also be employed a membrane prepared from thefollowing sulfur containing fluorinated vinylether compound: ##STR31##

The membrane to be used for lamination may preferably have a thicknessof 1/2 to 19/20 times the thickness of the entire laminated product inorder to make the electric resistance thereof smaller.

The above membrane can be reinforced in strength by backing with amechanical reinforcing material such as a net. As such backingmaterials, there may most suitably be used a net made ofpolytetrafluoroethylene fibers. A porous polytetrafluoroethylene sheetis also useful.

It is also possible to increase the strength of the membrane byincorporating polytetrafluoroethylene fibers during molding into amembrane. When a membrane with a laminated structure is employed, thereinforcing material may preferably be embedded on the side of themembrane with the greater copolymerization ratio of sulfur containingfluorinated vinylether compound. Reinforcing materials may be embeddedin the membrane by a method such as laminating, press contact embeddingor vacuum fusion embedding. For example, when a net is to be embedded, amembrane is placed on a net and the surface of the membrane opposite tothat contacted with the net is heated to a temperature no higher by 20°C. than the melting point of the membrane and the surface of themembrane contacted with the net maintained at a temperature higher by atleast 60° C. than the melting point of the membrane, while providing apressure difference between both sides of the membrane. The pressure onthe side contacted with the net is made lower than the opposite side.

Other than the above method, it is also possible to embed the net in themembrane after converting the exchange groups on the side opposite tothat contacted with the net to carboxylic acid esters.

The thickness of the membrane is generally 2500 microns or less,preferably 1000 micron or less, more preferably 500 microns or less. Thelower limit is restricted by the mechanical strength required, butusually 10 microns or more.

The copolymer of the present invention may be formed into particlesduring polymerization or molding according to conventional proceduresfor preparation of ion-exchange resins, and then subjected to thereaction used for converting a membrane into a fluorinated cationexchange membrane to provide fluorinated ion-exchange resin particles.

These ion-exchange resins can be processed into any desired shape suchas granules, membranes, fibers, strands, etc. By utilization of heatresistance and chemical resistance superior to hydrocarbon, type resins,they are useful generally in separation processes which are based onadsorption properties such as adsorptive separation of metallic ions orseparation of organic high molecular substances. They are also useful asacid catalysts for organic reactions.

The copolymer according to the present invention can also be used in theform of fibers or strands as ion-conductive reinforcing material for afluorinated cation exchange membrane.

Said copolymer may also be blended with other fluorocarbon type orhydrocarbon type copolymers to provide various blends useful for variouspurposes. It may also be provided as it is or as a mixture with asuitable solvent for use as lubricants, surfactants, etc. It is alsouseful as the starting material for a fluorinated elastomer.

The fifth object of the present invention is to provide a novelfluorinated cation exchange membrane for use in electrolysis of anaqueous alkali metal halide solution, comprising the following recurringunits (C), (D) and (E): ##STR32## (L is F, Cl, CF₃, OR_(F) or H, R_(F)being C₁ -C₅ perfluoroalkyl) ##STR33## (k is 0 or 1, l is an integer of3 to 5, M is H, metal or ammonium ion); ##STR34## (k and M are the sameas defined above and m is (l-1),

and having a carboxylic acid group density, which is defined as thepercentage of the number of carboxylic acid groups based on the totalnumber of all ion-exchange groups present in a layer substantiallyparallel to the surfaces of the membrane, of at least 20% on one surfaceof the membrane, said carboxylic acid group density being graduallydecreased toward the innerside of the membrane from said one surface ofthe membrane, and also a process for producing the same. In the abovecation exchange membrane, the relative proportion of the recurring units(C)/[(D)+(E)] may preferably be in the range from 1.5 to 14. It is alsopreferred that the density of carboxylic acid groups across the membraneshould be decreased moderately enough such that the gradient in terms ofthe decreased percentage of carboxylic acid groups per unit thicknessmay be 20%/micron at its maximum.

One specific feature of the membrane according to the present inventionresides in having excellent electrolysis performance of high currentefficiency and low electrolysis voltage. Another specific feature of themembrane resides in stability under more severe conditions than thoseusually employed, whereby said excellent electrolysis performance can bemaintained for a long time. The membrane can also be producedeconomically with ease and at low cost.

The excellent electrolysis performance of the membrane according to thepresent invention may be ascribed to the specific structure of themembrane, having a carboxylic acid group density on one surface of 20%to 100%, preferably 40% or more, more preferably 60% or more, saidcarboxylic acid group density gradually decreasing from said one surfacetoward the innerside of the membrane, i.e. in the direction of thicknessof the membrane. To give a quantitative expression of such a gradualdecrease of carboxylic acid group density from one surface of themembrane toward the depth of the membrane in terms of the maximumgradient, which is defined as the greatest decrease of carboxylic acidgroup density per unit thickness in the membrane, the maximum gradientshould preferably be 20 to 0.1% per one micron of the membranethickness, more preferably 10% to 1%. As a preferable structure, saidcarboxylic acid group density may reach substantially zero % at a depthof not more than 1/2 of the entire thickness of the membrane from onesurface. In other words, the carboxylic acid groups should preferably bepresent in the membrane locally in one half side of the membrane, beingmore enriched with a gradual increase the nearer to the surface on oneside, while the other half side of the membrane contains other exchangegroups, namely sulfonic acid groups. More preferably, the depth at whichthe carboxylic acid group density reaches zero % may be less than 1/2 ofthe entire thickness of the membrane, i.e. 1/4 or less, most preferably1/6 or less, to the lower limit of about 1μ.

When the membrane of the present invention is used for electrolysis ofan aqueous alkali metal halide solution, it is preferred to use themembrane with the surface having higher carboxylic acid group densityfacing toward the cathode. With such an arrangement, said surfaceshrinks when contacted with a highly concentrated alkali due to thepresence of carboxylic acid groups to increase the concentration offixed ions. As the result, permeation, migration and diffusion ofhydroxyl ions into the membrane can effectively be inhibited, wherebyhigh current efficiency can be exhibited.

The carboxylic acid group density on said one surface of the membranemay be variable depending on various factors such as the value of theratio (C)/[(D)+(E)], the current density, the temperature and the alkaliconcentration employed in electrolysis and can be optimally determinedby controlling the conditions in preparation. Generally speaking, as thevalue of (C)/[(D)+(E)] is greater, the carboxylic acid group density maybe lower.

On the other hand, according to a preferred embodiment of the membraneof the present invention, carboxylic acid groups are present primarilyin a thin layer on the side of one surface of the membrane, onlysulfonic acid groups being present in most of the residual portion. Forthis reason, the electric resistance in migration of alkali metal ionsfrom the anode chamber to the cathode chamber is extremely low ascompared with, for example, a membrane containing only carboxylic acidgroups. Due to the presence of sulfonic acid groups, the water contentin the membrane as a whole is also very large as compared with amembrane containing only carboxylic acid groups and therefore themembrane can be free from hardening or embrittlement due to shrinkage ofthe membrane even when used under severe conditions in a highlyconcentrated alkali for a long term.

One reason why the membrane of the present invention can be used morestably than the membrane of the prior art even under more severeconditions than those conventionally used may be ascribed to thespecific structure of the polymer substantially consisting of therecurring units (C), (D) and (E) as described above. For obtaining amembrane having high ion-exchange capacity as well as good physicaltoughness, it is preferred that the suffix k should be equal to zero,but there may also be partially mixed therewith a polymer wherein k isone. It is also preferred from the ease of preparation of the monomer,the physical properties of the resultant polymer and greater variablerange of the polymer properties that the suffix l should be equal to 3.A membrane with an l value of 6 or more is inferior to those with lvalues of 3 to 5 from the standpoint of difficulty in commercialproduction of the monomer and insufficient ion-exchange capacityobtained. A membrane wherein L is a fluorine atom is particularlypreferred from the aspects of heat resistance and chemical resistance.

The specific feature of the polymer structure as mentioned above isbased on the specific structure of the sulfur containing fluorinatedvinylether of the following formula used for preparation of the membraneof the invention: ##STR35## wherein k and l are the same as definedabove, Z is --S-- or --SO₂ --, R is C₁ -C₁₀ alkyl, an aryl Cl or C₁ -C₁₀perfluoroalkyl.

The above monomer is different in the structure of the terminal end orin the number of members of the ring in the product by-produced in thevinylization step, as compared with the sulfur containing fluorinatedvinylether of the formula: ##STR36## wherein n' is an integer of 0 to 2,which is used as starting material for a sulfonic acid type membrane ofthe prior art or a sulfonic acid type membrane having been formed bychemical treatment of carboxylic acid groups in the surface stratumthereof, and therefore it is possible to form substantially no or todecrease to a great extent the cyclization reaction in the vinylizationstep as mentioned above. Thus, a monomer with k=0 can easily be preparedand there is also no deterioration of polymer properties due tocyclization during polymerization.

Accordingly, since it is possible to use a monomer with k=0 as principalstarting material for preparation of a membrane, the resultant polymercan have a structure containing substantially no or only a very smallproportion of pendant groups: ##STR37## Consequently, with the samelevel of the ion-exchange capacity, the content of fluorinated olefincan be increased. In other words, there can be produced a physicallytough membrane with enhanced ion-exchange capacity. Moreover, while itsmechanism has not yet been clarified, such a membrane can maintainstable performance, being prevented from encountering problems ofpeel-off or crack of the carboxylic acid layer, even when used undermore severe conditions than those conventionally used.

Another reason why the membrane of the present invention is stable undersevere conditions may be ascribed to the relative ratio of the recurringunits (C), (D) and (E), i.e. the ratio of (C)/[(D)+(E)] which isgenerally in the range from 1.5 to 14, preferably from 3 to 11, morepreferably from 3.5 to 6. When said ratio is less than 1.5, the membraneis liable to be swelled during usage and therefore cannot maintainstable performance for a long term. On the other hand, if it is greaterthan 14, the membrane is liable to shrink so as to make the electricresistance of the membrane impractically high.

The ion-exchange capacity of the membrane according to the presentinvention may be represented by the following formula as being dependenton the structure of the recurring units, the ratio of recurring unitsand the carboxylic acid group density: ##EQU1## wherein r=(C)/[(D)+(E)],M_(L) is the molecular weight of the atomic group L and d is thecarboxylic acid group density, k, l and m being the same as definedabove.

In the prior art, the ion-exchange capacity of an ion-exchange capacityhas been indicated in specific numerical values, as disclosed byJapanese published unexamined patent application Nos. 120492/1975,130495/1976, 36589/1977 and 24176/1977, and U.S. Pat. No. 4,065,366.According to the study by the present inventors, however, the swellingand shrinking behavior of a membrane with a given species ofion-exchange groups is not controlled by the ion-exchange capacity perse of the membrane but by the most important factors including thefluorinated olefin constituting the copolymer, the copolymer ratio ofthe fluorinated vinylether having ion-exchange groups and the presenceor absence of ##STR38## In order to obtain a membrane havingsufficiently low electric resistance and good physical toughness withsmall swelling or shrinking when used in electrolysis, it is required touse a fluorinated vinylether having no ##STR39## as principal componentsand to control the above copolymerization ratio within a certain range.The ion-exchange capacity as represented by the above formula is basedon such considerations.

It is not clear why the above copolymerization ratio has such a decisiveinfluence on the swelling and shrinking behavior of a membrane. Forconvenience of explanation, reference is made to a membrane containingthe most preferred fluorinated olefin, i.e. tetrafluoroethylene. Fromanalysis of X-ray diffraction of the membrane, tetrafluoroethylene seemsto be partially crystallized. Since the degree of crystallization isgreatly dependent on the above copolymerization ratio, it may beestimated that the crystallized, region will function asquasi-crosslinks which control the swelling and shrinking behavior ofthe membrane.

In the membrane according to the present invention, it is possible toprovide a structure containing substantially no or a small amount ofpendant groups: ##STR40## When a membrane with the same ion-exchangecapacity is to be prepared, the copolymerization ratio oftetrafluoroethylene can be increased in the membrane of the presentinvention, as compared with a membrane prepared by use of ##STR41## as asulfur containing fluorinated vinylether, thereby providing a membranehaving both high ion-exchange capacity and good physical toughness.

As described above, the membrane of the present invention is specific inhaving a carboxylic acid group density which is gradually decreased fromthe surface to the innerside, preferably at a gradient within a specificrange. This is still another reason why the membrane of the presentinvention is by far more stable than the membrane of the prior art undermore severe conditions than those conventionally used.

The membrane having a laminated structure comprising a membranecontaining carboxylic acid groups and a membrane containing sulfonicacid groups, as disclosed by Japanese published unexamined patentapplication Nos. 36589/1977 and 132089/1978, is incomplete in bonding aspreviously mentioned and liable to cause peel-off or water bubbles in ashort period of time at the laminated portion.

On the other hand, according to the experience of the present inventors,even when the carboxylic acid density can be controlled to a certainextent in a membrane having carboxylic acid groups formed by chemicaltreatment, as disclosed by Japanese published unexamined patentapplication Nos. 24176/1977, 104583/1978, 116287/1978 and 6887/1979, theresultant membrane is liable to cause peel-off or crack of thecarboxylic acid layer, as compared with the membrane of the presentinvention, presumably due to the problem in polymeric structure aspreviously mentioned.

In contrast, as illustrated in the Examples, the membrane of the presentinvention can maintain stable performance for by far a longer time thanthe membranes of the prior art without causing abnormal phenomena suchas peel-off or crack of the carboxylic acid layer even under theconditions of a high current density of 110 A/dm² and a high temperatureof 95° C. or higher.

The membrane of the present invention may also have laminated, on onesurface of the membrane with the lower carboxylic acid group density, afluorinated cation exchange membrane consisting substantially of theunits (C) as previously mentioned and the following recurring units (F):##STR42## wherein p"=0 or 1, q is an integer of 3 to 5, and M has thesame meaning as defined above, the ratio of recurring units being in thefollowing range:

    (C)/(F)<(C)/[(D)+(E)].

Such a structure is also preferred from the standpoint of lowering theelectric resistance of a membrane. In this case, in order to obtain amembrane having lower electric resistance with physical toughness, it ispreferred that p" may be equal to zero and q equal to l. It is alsopreferred that the thickness of the fluorinated cation exchange membranecomprising the recurring unit (F) may have a thickness 1/2 to 19/20 asthick as the entire membrane.

The membrane of the present invention may also be provided with abacking with a mechanical reinforcing material such as a net for thepurpose of increasing the strength of the membrane. As such a backingmaterial, a net made from polytetrafluoroethylene fibers is mostsuitable, but there may also be used a porous polytetrafluoroethylenesheet. It is also possible to incorporate fibrouspolytetrafluoroethylene during molding of a membrane for increasing thestrength thereof.

Referring now to the method for preparation of the membrane of thepresent invention, the membrane of the fluorinated copolymer used forpreparation of the membrane of the present invention can be producedaccording to the method as previously described. Then, as the secondstep, a part or all of the terminal groups of the recurring unit (G) ofa membrane prepared by the method as mentioned above comprisingessentially the recurring units (C) and (G) as shown below: ##STR43## (Lis the same as defined above) ##STR44## (k, l, Z and R are the same asdefined above)

are converted, if necessary, to sulfonylchloride groups --CF₂ SO₂ Cl orsulfonylbromide groups --CF₂ SO₂ BR, preferably sulfonylchloride groupsusing a halogenating agent represented by the formula:

    B.sub.2 (AB.sub.d-2).sub.e

wherein A is P; B is Cl or Br and d=3 or 5; and e=0 or 1. The reactionused in this step is different depending on the types of Z and R.Details of each reaction are set forth below for each type.

(a) When Z=--S--:

It is generally possible to react a halogen with a membrane forconversion to sulfonyl halide groups. From the standpoint of reactivityand ease of handling, chlorine may preferably be used. In this case,--CF₂ SO₂ Cl is formed directly or via --CF₂ SCl. The reactionconditions may be variable within a broad range, but the reactiontemperature is generally from 0° to 300° C. under normal pressure orunder pressurization. The chlorine employed may either be in the drystate or in a solution dissolved in water, an organic solvent such asacetic acid, trichloroacetic acid, trifluoroacetic acid, or an inorganicsolvent such as S₂ Cl₂.

When Z=--S--, it may also be oxidized into sulfone of Z=--SO₂ -- orsulfoxide of Z=--SO--, using an oxidizing agent conventionally used suchas ozone, conc.sulfuric acid, fuming sulfuric acid, nitric acid,sulfuryl chloride, hydrogen peroxide, potassium permanganate orpotassium dichloromate. Said oxidation treatment may be conductedusually in an aqueous solution at 20° to 200° C., whereby an organicsolvent such as acetic acid or trichloroacetic acid may also be presentin the solution to accelerate permeation of the oxidizing agent into themembrane. The sulfoxide formed by the above oxidation treatment may beconverted to --CF₂ SO₂ Cl with chlorine.

(b) When Z=--SO₂ -- (sulfone)

Conversion to sulfonylchloride groups is possible according to themethod similar to that used in the case of Z=--S--. It may also beconverted to sulfonic acid groups --CF₂ SO₃ M by hydrolysis with analkali. The hydrolysis may be carried out using a solution of causticsoda or caustic potash dissolved in water, a mixed solvent of water withan organic solvent such as alcohol or dimethylsulfoxide, optionallycontaining an oxidizing agent added, for example, at 20° to 200° C.

The thus obtained sulfonic acid groups may easily be converted tosulfonylchloride groups by reaction with vapors of phosphoruspentachloride or a solution of phosphorus pentachloride dissolved inphosphorus oxychloride, an organic halide compound, etc. according tothe method and the conditions as described in Japanese publishedunexamined patent application Nos. 134888/1977 and 4289/1979. A mixtureof phosphorus trichloride with chlorine may also be used.

Further, as the third step, a part or all of the sulfonyl halide groupsat the terminal end of the recurring unit (H): ##STR45## wherein k and lare the same as defined above, X" is Cl or Br, preferably Cl, areconverted to carboxylic acid groups. From the standpoint of reactivityand ease in handling, it is most preferable to use sulfonylchloridegroups.

Such a conversion can be accomplished by treatment of a membranecomprising the recurring units (C) and (H) with a reducing agent andaccording to the reaction method and reaction conditions as geneallydescribed in Japanese published unexamined patent application Nos.24176/1977, 24177/1977 and 132094/1978, thereby converting --CF₂ --directly bonded to sulfur atom directly or via sulfinic acid groups intocarboxylic acid groups. As the result, there is formed a specificstructure of m=(l-1) in the pendant groups of the recurring units (E).

The reducing agents to be used in the present invention may preferablybe selected from acids having reducing ability such as hydroiodic acid,hydrobromic acid, hypophosphorous acid, hydrogen sulfide water, arsenousacid, phosphorous acid, sulfurous acid, nitrous acid, formic acid,oxalic acid, etc., their metal salts, ammonium salts, and hydrazines,from the standpoint of reactivity and ease in handling. Among them, aninorganic acid having reducing ability is most preferred. These reducingagents may be used alone or, if necessary, as a mixture.

The structure of the membrane comprising carboxylic acid groups enrichedon only one surface of the membrane, which is the excellent specificfeature of the membrane according to the present invention, may berealized easily by applying the second step reaction or preferably thethird step reaction on one surface of the membrane. In case of amembrane having a laminated structure, these reactions may be applied onthe surface opposite to that on which lamination is effected.

The gradient of the carboxylic acid group density may be controlled to adesired shape of the density curve by adequately controlling variousfactors in the reactions in the second or the third step such astemperature, time, pressure, solvent composition, etc. to therebybalance the reaction rate and the diffusion velocity of a reagent intothe membrane. For ease of control, it is preferred to effect suchcontrolling in the third step.

As a preferable method for controlling the carboxylic acid groupdensity, there may be mentioned a method wherein the above treatmentwith a reducing agent is effected in the presence of at least oneorganic compound having 1 to 12 carbon atoms selected from alcohols,carboxylic acids, sulfonic acids, nitriles or ethers, using especially asolution of said organic compounds dissolved in an aqueous reducingagent solution. In particular, carboxylic acids may preferably be usedas such organic compounds. These organic compounds may be added in anamount, which is variable depending on the membrane employed, thereducing agent and organic compound employed as well as the reactionconditions and may suitably be selected within the range of 100 ppm ormore.

Examples of alcohols to be used in the present invention may includemethanol, ethanol, propanol, ethylene glycol, diethylene glycol,1,4-butane diol, 1,8-octane diol, glycerine, and the like.

As typical examples of carboxylic acids and sulfonic acids, there may bementioned formic acid, acetic acid, propionic acid, butyric acid,iso-butyric acid, n-valeric acid, caproic acid, n-heptanoic acid,caprylic acid, lauric acid, fluoroacetic acid, chloroacetic acid,bromoacetic acid, dichloroacetic acid, malonic acid, glutaric acid,trifluoroacetic acid, perfluoropropionic acid, perfluorobutyric acid,perfluorovaleric acid, perfluorocaproic acid, perfluoro-n-heptanoicacid, perfluorocaprylic acid, perfluoroglutaric acid, trifluoromethanesulfonic acid, perfluoroheptane sulfonic acid, methane sulfonic acid,ethane sulfonic acid, propane sulfonic acid, butane sulfonic acid,pentane sulfonic acid, hexane sulfonic acid, heptane sulfonic acid, andso on. Preferably, acetic acid, propionic acid, caprylic acid,trifluoroacetic acid, perfluorocaprylic acid or perfluorobutyric acidmay be used.

Typical examples of nitriles are acetonitrile propionitrile,adiponitrile, and the like. Ethers may be exemplified by diethylether,tetrahydrofuran, dioxane, ethylene glycol dimethylether, diethyleneglycol dimethyl ether, etc. Among these organic compounds, somecompounds may undergo chemical changes depending on the reducing agentemployed and therefore it is desired to avoid such a combination.

The gradient of the carboxylic acid group density in the membrane may bedetermined, as illustrated in the Examples, by staining thecross-section of a membrane with a suitable dye and observing the resultof staining, or alternatively by scraping the membrane substantially inparallel to the surface thereof(usually in a thickness of about 1 to 5micron per each scraping), subjecting the scraped face to attenuatedtotal reflection (hereinafter referred to as ATR) and calculating fromthe changes in intensity of the absorption peak based on the carboxylicacid groups.

In the membrane of the present invention or other fluorinated cationexchange membranes, the pendant structure having bonded ion-exchangegroups can be identified by measurement of ATR or IR absorption spectrumafter the reaction for elimination of ion-exchange groups.

Other than the method as described above wherein a reducing agent isused, there may also be used the same method as described in Japanesepublished unexamined patent application No. 125986/1978, whereinsulfonyl halide groups are once converted to --CF₂ I, followed byconversion to carboxylic acid groups. Alternatively, the membranecomprising the recurring units (G) may be irradiated with ultra-violetrays or an electron beam to be directly converted to carboxylic acidgroups. It is also possible to obtain a membrane containing carboxylicacid groups with more --CF₂ -- than that obtained by use of a reducingagent, i.e. m being greater than (l-1) in the pendant groups of therecurring unit (E), according to the method as described in Japanesepublished unexamined patent application Nos. 104583/1978 and116287/1978. Said method comprises reacting a membrane having sulfonylhalide groups or a membrane having sulfinic acid groups or --CF₂ Iobtained as intermediate in the method as described above with acompound having carbonyl groups or unsaturated bonding under theconditions to eliminate SO₂ or iodine atom ionically or radically.According to these methods, however, it is very difficult to control thegradient of the carboxylic acid density; many steps are required for thereaction; the cost is high; expensive reagents are necessary; sidereactions can be suppressed only with difficulty; pendant groups cannotbe in the form of perfluoro groups; or the membrane may be damagedphysically during the treatment. In any of these respects, any of saidalternative method is inferior to the methods wherein a reducing agentis used. For this reason, in preparation of a membrane to be used undermore severe conditions than those conventionally used, it is morepreferable to use the method employing a reducing agent than thosealternative methods as mentioned above.

The fourth step for preparation of the membrane of the present inventionis to convert all of the residual sulfur containing terminal groups tosulfonic acid groups. This can easily be done according to the reactionas mentioned in the second step reaction or by application of thereactions such as oxidation, hydrolysis, etc. as described in Japanesepublished unexamined patent application Nos. 24176/1977 and 24177/1977.

As apparently seen from the preparation methods as described above, themembrane of the present invention can be derived from common startingmaterials according to simple reactions to have carboxylic acid groupsand sulfonic acid groups. Thus the membrane can be manufactured easilyand advantageously at low cost.

The cation exchange membrane according to the present invention mayfavorably be employed in electrolysis of an aqueous alkali metal halidesolution. That is, the membrane of the present invention is useful notonly in electrolysis of an alkali metal halide under conventionalelectrolysis conditions, i.e. a current density of 10 to 70 A/dm², atemperature of 20° to 100° C., alkali metal halide concentration of 1 to5N and an alkali concentration of 1 to 15N, but is also useful undersevere conditions, i.e. a current density of 70 to 200 A/dm² and atemperature of 100° to 150° C., with stable performance for a long time.

The sixth object of the present invention is to provide a novelfluorinated cation exchange membrane containing sulfonic acid groups,comprising essentially the following units (I) and (J): ##STR46##wherein k is 0 or 1, l is an integer of 3 to 5 and M is H, a metal orammonium ion, the ratio of the numbers of the recurring units (I) and(J) being (I)/(J)=1.5 to 14. For the purpose of the present invention,it is preferred that the suffix k should be equal to zero. It is alsopreferred that the suffix l should be equal to 3 for ease in preparationof the monomer and a greater variable range of the polymer composition.A membrane with an l value of 6 or more is inferior to those with lvalues of 3 to 5 from the standpoint of difficulty in synthesis of themonomer and insufficient ion-exchange capacity obtained. Further, theratio (I)/(J) may preferably in the range from 3 to 11, particularlypreferably from 3.5 to 6.

The above sulfonic acid type cation exchange membrane can be prepared byuse of the membrane of the aforesaid fluorinated copolymer as describedabove. Said membrane can be treated by application of the reactions asdescribed above to convert all of the sulfur containing terminal groupsinto sulfonic acid groups to give the novel cation exchange membranecontaining sulfonic acid groups comprising the recurring units (I) and(J) as defined above.

This membrane is useful in various fields such as electrolysis of anaqueous alkali metal halide solution, electrolysis of water, diaphragmsfor fuel cells, etc. For the reason mentioned below, this membrane issuperior to fluorinated cation exchange membranes containing sulfonicacid groups conventionally used in commercial application.

The specific feature in performance of the sulfonic acid type membraneaccording to the present invention is based on the specific structure ofthe sulfur containing fluorinated vinylether of the following formulaused for preparation of said sulfonic acid type membrane ##STR47##wherein k, l, Z and R are the same as defined above.

The above monomer is different in the structure of the terminal end orin the number of members of the ring, as compared with the sulfurcontaining fluorinated vinylether of the formula: ##STR48## wherein n'is 0 to 2, which is used as starting material for a sulfonic acid typemembrane of the prior art, and therefore it is possible to formsubstantially no or to decrease to a great extent the cyclizationreaction in the vinylization step as mentioned above. Thus, a monomerwith k=0 can easily be prepared and there is also no deterioration ofpolymer properties due to cyclization during polymerization.

Accordingly, since it is possible to use a monomer with k=0 as principalstarting material for preparation of a membrane, the resultant polymercan have a structure containing substantially no or only a very smallproportion of pendant groups: ##STR49## Consequently, with the samelevel of the ion-exchange capacity, the content of fluorinated olefincan be increased. In other words, there can be produced a physicallytough membrane with enhanced ion-exchange capacity.

Another reason why the membrane of the present invention is stable undersevere conditions may be ascribed to the relative ratio of the recurringunits (I) and (J), i.e. the ratio of (I)/(J) which is generally in therange from 1.5 to 14, preferably from 3 to 11, more preferably from 3.5to 6. When said ratio is less than 1.5, the membrane is liable to swellduring usage and therefore cannot maintain a stable performance for along term. On the other hand, if it is greater than 14, the membrane isliable to shrink so as to make the electric resistance of the membraneimpractically high.

The ion-exchange capacity of the membranes of the present invention maybe represented by the following formula as being dependent on thestructure of the recurring units, and the ratio of recurring units:##EQU2## wherein r=(I)/(J), k and l are the same as defined above.

In the prior art, the ion-exchange capacity of an ion-exchange membraneshas been indicated in specific numerical values, as disclosed byJapanese published unexamined patent application Nos. 120492/1975,130495/1976, 36589/1977 and 24176/1977, and U.S. Pat. No. 4,065,366.According to the study by the present inventors, however, as notedabove, the swelling and shrinking behavior of a membrane with a givenspecies of ion-exchange groups is not controlled by the ion-exhangecapacity per se of the membrane but by the most important factorsincluding the fluorinated olefin constituting the copolymer, thecopolymer ratio of the fluorinated vinylether having ion-exchange groupsand the presence or absence of ##STR50## In order to obtain a membranehaving sufficiently low electric resistance and good physical toughnesswith small swelling or shrinking when used in electrolysis, it isrequired to use a fluorinated vinylether having no ##STR51## asprincipal component and to control the above copolymerization ratiowithin a certain range. The ion-exchange capacity as represented by theabove formula is based on such considerations.

It is not clear why the above copolymerization ratio has such a decisiveinfluence on the swelling and shrinking behavior of a membrane. Forconvenience of explanation, reference is made to a membrane containingthe most preferred fluorinated olefin, i.e. tetrafluoroethylene. Fromanalysis of X-ray diffraction of the membrane, tetrafluoroethylene seemsto be partially crystallized. Since the degree of crystallization isgreatly dependent on the above copolymerization ratio, it may beestimated that the crystallized region will function as quasi-crosslinkswhich control the swelling and shrinking behavior of the membrane.

In the membrane according to the present invention, it is possible toprovide a structure containing substantially no or only a small amountof pendant groups: ##STR52## When a membrane with the same ion-exchangecapacity is to be prepared, the copolymerization ratio oftetrafluoroethylene can be increased in the membrane of the presentinvention, as compared with a membrane prepared by use of ##STR53## as asulfur containing fluorinated vinylether, thereby providing a membranehaving both high ion-exchange capacity and good physical toughness.

In the above sulfonic acid type membrane or other fluorinated cationexchange membranes, the pendant structure having bonded ion-exchangegroups can be identified by measurement of ATR or IR absorption spectrumafter the reaction for elimination of ion-exchange groups.

The fluorinated cation exchange membrane having sulfonic acid groups canbe prepared from a membrane prepared by molding of a copolymer obtainedby polymerization by converting the terminal groups of the recurringunits (G) of a membrane prepared by the method as described abovecomprising essentially the recurring units (C) and (G) as shown below:##STR54## (L is the same as defined above) ##STR55## (k, l, Z and R arethe same as defined above) to sulfonylchloride groups --CF₂ SO₂ Cl orsulfonylbromide groups --CF₂ SO₂ Br, preferably sulfonylchloride groups,using a halogenating agent represented by the formula:

    B.sub.2 (AB.sub.d-2).sub.e

wherein A is P; B is Cl or Br and d=3 or 5 and e=0 or 1. The reactionused in this step may be carried out according to the method and underthe conditions as already described.

The sulfonyl halide groups formed in the above method may readily beconverted to sulfonic acid groups by hydrolysis with an alkali. In thiscase, the reaction may be accelerated by use of an organic solvent suchas methanol, ethanol, dimethylsulfoxide, etc.

The thus prepared sulfonic acid type membrane can also be modified tohave lower electric resistance by lamination with a membrane havinggreater exchange capacity or improved in physical strength by embeddinga suitable reinforcing material, as previously described, with amembrane having both carboxylic acid groups and sulfonic acid groups.

The present invention is illustrated in further detail by referring tothe following Examples, by which the present invention is not limited.

EXAMPLE 1

In a stainless steel autoclave of 3-liter capacity, there are charged250 g of sodium ethyl mercaptide, 530 g of dimethyl carbonate and 750 gof tetrahydrofuran, and then the reaction system is brought into areduced pressure of 50 to 60 mm Hg. While maintaining the temperature at15° C. under vigorous agitation of the reaction system,tetrafluoroethylene is gradually blown into the system under reducedpressure. With the progress of the reaction, the rate oftetrafluoroethylene consumed is lowered until, finally at thetetrafluoroethylene pressure of 1 kg/cm², there is no more consumptionof tetrafluoroethylene. After the reaction, the reaction mixture isneutralized with 300 g of 98% sulfuric acid. The sodium sulfate formedis filtered off and the filtrate is previously evaporated by anevaporator to remove tetrahydrofuran, followed by distillation of theresidue, to obtain 520 g of the fraction of distillate at 84° C./30 mmHg. Said fraction is found to have the structure of C₂ H₅ SCF₂ CF₂COOCH.sub. 3 from elemental analysis, IR and NMR spectra.

IR characteristic absorption (liquid): 2960, 2930, 2870 cm⁻¹ (C₂ H₅ --),1780 cm⁻¹ (--CO₂ --), 1300-1100 cm⁻¹ (--CF₂ --).

Elemental analysis: C₆ H₈ F₄ O₂ S: Calculated: C, 32.7; H, 3.6; F, 34.5;S, 14.5. Found: C, 32.2; H, 3.9; F, 33.9; S, 14.3.

EXAMPLE 2

While heating 100 g of the compound C₂ H₅ SCF₂ CF₂ COOCH₃ obtained inExample 1 at 50° C., an aqueous 10N caustic soda solution is addedgradually dropwise thereto and the dropwise addition is continued untilthe reaction system is weakly alkaline to convert said compound into C₂H₅ SCF₂ CF₂ CO₂ Na. After removing sufficiently the methanol formed inthe reaction system by an evaporator, the reaction system is made weaklyacidic by addition of conc. sulfuric acid. From the reaction systemseparated into two layers, the organic layer comprising C₂ H₅ SCF₂ CF₂CO₂ H is separated, followed by thorough drying of said organic layer.In a stainless steel autoclave, there are charged 80 g of C₂ H₅ SCF₂ CF₂CO₂ H, 40 cc of 1,1,2-trichloro-1,2,2trifluoroethane and 32 g of sodiumfluoride, and then 63 g of sulfur tetrafluoride is pressurized into saidautoclave. While stirring the mixture, the reaction is carried out at80° C. for 4 hours. After completion of the reaction, gas purge iseffected with dry nitrogen and sodium fluoride is filtered off from thereaction mixture. The filtrate is subjected to distillation to give 54 gof the fraction of distillate at 46° C./100 mm Hg.

Said fraction is identified by elemental analysis, IR and NMR spectra tohave the structure of C₂ H₅ SCF₂ CF₂ COF.

IR characteristic absorption (liquid): 2960, 2930, 2870 cm⁻¹ (C₂ H₅),1880 cm⁻¹ (--COF), 1300-1100 cm⁻¹ (--CF₂ --).

Elemental analysis values: C₅ H₅ F₅ OS. Calculated: C, 28.8; H, 2.4; F,45.7; S, 15.4. Found: C, 29.0; H, 2.6; F, 45.2; S, 15.3.

EXAMPLE 3

The compound C₂ H₅ SCF₂ CF₂ CO₂ H (80 g) prepared, in Example 2, bysubjecting the compound C₂ H₅ SCF₂ CF₂ COOCH₃ to the alkali treatmentand to the conc. sulfuric acid treatment, is mixed with 400 ml of amixture (2:1, volume ratio) of 30% aqueous hydrogen peroxide solutionand glacial acetic acid. The reaction is carried out with stirring at90° C. for 5 hours.

To the resultant reaction mixture, there is added conc. sulfuric acid toseparate the mixture into two layers, from which the organic layercomprising C₂ H₅ SO₂ CF₂ CF₂ CO₂ H is separated. To this layer is addedmethanol under acidic conditions, and the reaction is conducted at 60°C. for 3 hours. Then, the reaction mixture is subjected to distillationto give 70 g of the fraction of distillate at 183°-186° C./40 mm Hg.Said fraction is identified by elemental analysis, IR and NMR spectra tohave the structure of C₂ H₅ SO₂ CF₂ CF₂ COOCH₃.

IR characteristic absorption (liquid): 2960, 2930, 2870 cm⁻¹ (--C₂ H₅),1780 cm⁻¹ (--CO₂ --), 1360 cm⁻¹ (--SO₂ --), 1300-1100 cm⁻¹ (--CF₂ --).

Elemental analysis values: C₆ H₈ F₄ O₄ S; Calculated: C, 28.6; H, 3.2;F, 30.2; S, 12.7. Found: C, 28.3; H, 3.6; F, 29.7; S, 12.9.

EXAMPLE 4

After drying thoroughly the organic layer comprising C₂ H₅ SO₂ CF₂ CF₂CO₂ H prepared in Example 3, 100 g of said organic layer, 50 cc of1,1,2-trichloro-1,2,2-trifluoroethane and 40 g of sodium fluoride arecharged into an autoclave of 500 ml capacity, followed by pressurizationof 100 g of sulfur tetrafluoride thereinto. While stirring the mixture,the reaction is carried out at 80° C. for 6 hours. After the reaction isover, dry nitrogen is flushed for gas purge and sodium fluoride isfiltered off from the reaction mixture. Distillation of the filtrategives 90 g of the fraction of distillate at 59°-65° C./13 mm Hg.

Said fraction is identified by elemental analysis, IR and NMR spectra tohave the structure of C₂ H₅ SO₂ CF₂ CF₂ COF.

IR characteristic absorption (liquid): 2960, 2930, 2870 cm⁻¹ (--C₂ H₅),1880 cm⁻¹ (-COF), 1360 cm⁻¹ (--SO₂ --), 1300-1100 cm⁻¹ (-CF₂ --).

Elemental analysis values: C₅ H₅ F₅ O₃ S: Calculated: C, 25.0; H, 2.1;F, 39.6; S, 13.3. Found: C, 25.5; H, 1.8; F, 39.2; S, 13.1.

EXAMPLE 5

In a stainless steel autoclave of 3-liter capacity, there are charged280 g of sodium methyl mercaptide, 530 g of dimethyl carbonate and 1000g of tetrahydrofuran, and then the reaction system is brought into areduced pressure of 50 to 60 mm Hg. While vigorously agitating thereaction system and maintaining the temperature at 10° C.,tetrafluoroethylene is gradually blown into the system under reducedpressure. With progress of the reaction, the rate of tetrafluoroethyleneconsumed is lowered. Finally, at the tetrafluoroethylene pressure of 1kg/cm², there is no more consumption of tetrafluoroethylene. After thereaction, the reaction mixture is neutralized with 380 g of conc.sulfuric acid(98%). The sodium sulfate formed is filtered off and thefiltrate is previously evaporated by an evaporator to removetetrahydrofuran. Distillation of the residue gives 660 g of the fractionof distillate at 83° C./50 mm Hg.

Said fraction is identified by elemental analysis. IR and NMR spectra tohave the structure of CH₃ SCF₂ CF₂ COOCH₃.

IR characteristic absorption (liquid): 3025, 2970, 2850 cm⁻¹ (CH₃ --),1780 cm⁻¹ (--CO₂ --), 1300-1100 cm⁻¹ (--CF₂ --).

Elemental analysis values: C₅ H₆ F₄ O₂ S. Calculated: C, 29.1; H, 2.9;F, 36.9; S, 15.5. Found: C, 29.5; H, 2.4; F, 36.1; S, 15.7.

EXAMPLE 6

While heating 100 g of the compound CH₃ SCF₂ CF₂ COOCH₃ at 50° C., 10N-aqueous caustic soda solution is added gradually dropwise until thereaction system is weakly alkaline to convert said compound to CH₃ SCF₂CF₂ CO₂ Na. After complete removal of the methanol formed in thereaction system, conc. sulfuric acid is added to the reaction system tomake it acidic. From the reaction system separated into two layers, theorganic layer comprising CH₃ SCF₂ CF₂ CO₂ H is separated and saidorganic layer is thoroughly dried. In an autoclave of stainless steel,there are charged 80 g of CH₃ SCF₂ CF₂ CO₂ H, 40 cc of1,1,2-trichloro-1,2,2-trifluoroethane and 32 g of sodium fluoride, andthen 65 g of sulfur tetrafluoride is pressurized into said autoclave.While stirring the mixture, the reaction is carried out at 80° C. for 4hours. After the reaction is over, dry nitrogen is flushed for gas purgeand the reaction mixture is filtered to remove sodium fluoride. Thefiltrate is distilled to give 57 g of the fraction of distillate at74°-76° C. Said fraction is identified by elemental analysis, IR and NMRspectra to have the structure of CH₃ SCF₂ CF₂ COF.

IR characteristic absorption (liquid): 3025, 2970, 2850 cm⁻¹ (CH₃ --),1880 cm⁻¹ (--COF), 1300-1100 cm⁻¹ (--CF₂ --).

Elemental analysis values: C₄ H₃ F₅ OS: Calculated: C, 24.7; H, 1.5; F,49.0; S, 16.5. Found: C, 24.9; H, 1.8; F, 48.2; S, 16.3.

EXAMPLE 7

The compound CH₃ SCF₂ CF₂ COOH (100 g) prepared, in Example 6, bysaponifying CH₃ SCF₂ CF₂ COOCH₃, followed by acid treatment and dryingtreatment, is introduced into a reactor. While maintaining thetemperature in the reactor at 80 to 85° C. under vigorous agitation,there are gradually added drops of a mixture (60 cc) of thionylchloride-dimethylformamide(thionyl chloride/dimethylformamide=20/1,volume ratio). After completion of the dropwise addition, the reactionis continued until generation of hydrogen chloride gas is terminated. Ontermination of hydrogen chloride gas generation, the reaction mixture isdistilled to give 110 g of the fraction of distillate boiling at103°-105° C. (principally composed of CH₃ SCF₂ CF₂ COCl).

In a reactor, there are charged 140 g of NaF and 100 cc of drytetramethylene sulfone. After heating the mixture to 85° C., undervigorous agitation, the above CH₃ SCF₂ CF₂ COCl (110 g) is addedgradually dropwise into the mixture. After the reaction has continuedfor one hour, a vacuum line equipped with a cooling trap is connected tothe reactor to reduce the pressure in the reactor to 10 mm Hg andheating is effected at 100° C. for 30 minutes. The condensed liquidproduct in the trap is distilled to give 80 g of the fraction ofdistillate at 74°-76° C.

Said fraction is identified by elemental analysis, IR and NMR spectra tohave the structure of CH₃ SCF₂ CF₂ COF.

IR characteristic absorption (liquid): 3025, 2970, 2850 cm⁻¹ (CH₃ --),1880 cm⁻¹ (--COF), 1300-1100 cm⁻¹ (--CF₂ --).

Elemental analysis values: C₄ H₃ F₅ OS: Calculated: C, 24.7; H, 1.5; F,49.0; S, 16.5. Found: C, 24.5; H, 1.7; F, 48.6; S, 16.9.

EXAMPLE 8

The compound C₂ H₅ SCF₂ CF₂ COOCH₃ prepared in Example 1 (330 g) isadded dropwise at room temperature over one hour, while under vigorousagitation, into a reactor wherein chlorine gas (500 ml/minute) ispreviously passed through trifluoroacetic acid (100 ml). After saiddropwise addition, the reaction mixture is left to stand for 10 hours,followed by distillation of the product and collection of the fractionof distillate at 70°-75° C./60 mm Hg to give 310 g of said fraction ofdistillate.

Said fraction is identified by elemental analysis, IR spectrum, NMRspectrum and to have the formula ClSCF₂ CF₂ CO₂ CH₃.

Elemental analysis values: Found: C, 21.4; H, 1.2; F, 33.1; S, 13.9.Calculated (for C₄ H₃ F₄ SO₂ Cl): C, 21.2; H, 1.3; F, 33.5; S, 14.1.

EXAMPLE 9

While passing chlorine gas at the rate of 500 ml/minute into a coldwater (200 ml) previously saturated with chlorine, under vigorousagitation, the sulphenyl chloride prepared in Example 8 (226.3 g) isadded gradually thereto. After the addition is completed, the reactionis continued for an additional 5 hours. Then, the lower layer is takenout to obtain 232 g of the fraction of distillate at 80°-82° C. under 60mm Hg. Said fraction is identified by IR spectrum, elemental analysisand NMR spectrum to have the structure of ClSO₂ CF₂ CF₂ CO₂ CH₃.

IR absorption spectrum: ##STR56## 1785 cm⁻¹ (--COOCH₃), 2960 cm⁻¹(--CH₃).

Elemental analysis: Found: C, 18.7; H, 1.0; F, 29.1; S, 12.6. Calculated(for C₄ H₃ F₄ SO₄ Cl): C, 18.6, H, 1.2, F, 29.4; S, 12.4.

EXAMPLE 10

The perfluoro-3-chlorosulfonylmethyl propionate (258.5 g) obtained inExample 9 is neutralized with 8N-NaOH, followed by removal of water andmethanol.

After the residue is dried, phosphorus pentachloride (312 g) andphosphorus oxychloride (150 g) are added thereto and the reaction iscarried out under reflux on a heating bath at 130° C. for 10 hours.After the reaction, distillation of the product gives 220 g of thefraction of distillate at 70° C. under 100 mm Hg.

This substance is identified by IR absorption spectrum, elementalanalysis and NMR spectrum to be ClSO₂ CF₂ CF₂ COCl(perfluoro-3-chlorosulfonylpropionyl chloride).

IR absorption spectrum: 1790 cm⁻¹ (--COCl), 1415 cm⁻¹ (--SO₂ Cl).

Elemental analysis: Found: C, 13.4; F, 28.5; S, 12.1; Cl, 27.3.Calculated (for C₃ F₄ SO₃ Cl₂): C, 13.7; F, 28.9; S, 12.2; Cl, 27.0.

EXAMPLE 11

In a stainless steel autoclave of 500 cc capacity equipped with a gasblowing inlet, there are charged 100 g of the compound C₂ H₅ SCF₂ CF₂COF prepared similarly as in Example 2, 120 g oftetraglyme(tetraethyleneglycol dimethylether) and 75 g of dry CsF. Afterthe mixture is left to stand at room temperature for 16 hours withstirring, 80 g of hexafluoropropylene oxide (hereinafter referred to asHFPO) is blown into the autoclave while maintaining the temperature at30° C., gradually while maintaining the pressure at 1.5 kg/cm² or lower.After a predetermined amount of HFPO is blown into the autoclave,stirring is conducted to a constant pressure and unaltered HFPO isthereafter removed. The residue is subjected to distillation, whereby 70g of the fraction of distillate at 84°-87° C./100 mm Hg is obtained. Thefraction is found to have the structure of ##STR57## as identified byelemental analysis, IR and NMR spectra.

IR (liquid): 2960, 2930, 2870 cm⁻¹ (--C₂ H₅), 1880 cm⁻¹ (--COF),1100-1300 cm⁻¹ (--CF₂ --).

Elemental analysis: C₈ H₅ F₁₁ O₂ S: Calculated: C, 25.7, H, 1.3; F,55.9; S, 8.6. Found: C, 26.1; H, 1.5; F, 54.8; S, 8.7.

EXAMPLE 12

The reaction is conducted under the same conditions as in Example 11except that the amount of HFPO is changed to 160 g and the reactiontemperature to -10° C. After the reaction, distillation of the productis carried out to give the following fractions of distillate:

    ______________________________________                                         ##STR58##            (b.p. 84-87° C./ 100 mm Hg)                                                        20 g                                         ##STR59##            (115-125° C./ 100 mm Hg)                                                           50 g                                         ##STR60##            (n ≧ 2)                                                                            50 g                                        ______________________________________                                    

The structure of each fraction of distillate is identified by IR spectraand measurement of molecular weight by titration.

EXAMPLE 13

When the same procedure as in Example 11 is repeated except for using100 g of C₂ H₅ SO₂ CF₂ CF₂ COF as prepared in Example 4 in place of C₂H₅ SCF₂ CF₂ COF, there is obtained 50 g of the fraction of distillate at90°-95° C./10 mm Hg. Said fraction is identified by elemental analysis,IR and NMR spectra to have the structure of ##STR61##

IR (liquid): 2960, 2930, 2870 cm⁻¹ (--C₂ H₅), 1880 cm⁻¹ (--COF), 1360cm⁻¹ (--SO₂ --), 1100-1300 cm⁻¹ (--CF₂ --).

Elemental analysis: C₈ H₅ F₁₁ O₄ S: Calculated: C, 23.6: H, 1.2; F,51.5; S, 7.9. Found: C, 24.0; H, 1.4; F, 50.4; S, 8.0.

EXAMPLE 14

In a 500 cc autoclave made of stainless steel equipped with a gasblowing inlet, there are charged 100 g of the compound CH₃ SCF₂ CF₂ COFprepared in Example 7, 57 g of tetraglyme(tetraethyleneglycoldimethylether) and 39 g of CsF. After the mixture is left to stand atroom temperature with stirring for 16 hours, 104 g ofhexafluoropropylene oxide (hereinafter referred to as HFPO) is blowninto the autoclave gradually while maintaining the pressure at 1.5kg/cm² or lower, while maintaining the temperature at 5° C. After apredetermined amount of HFPO is charged, stirring is conducted to aconstant pressure and then unaltered HFPO is removed. After separatingCsF from the reaction mixture by filtration, the filtrate is distilledto give 65 g of the fraction of distillate at 69°-72° C./100 mm Hg. Saidfraction is identified by elemental analysis, IR and NMR spectra to havethe structure of ##STR62##

IR characteristic absorption (liquid): 3025, 2970, 2850 cm⁻¹ (--CH₃),1880 cm⁻¹ (--COF), 1300-1100 cm⁻¹ (--CF₂ --).

Elemental analysis: C₇ H₃ F₁₁ O₂ S: Calculated: C, 23.3; H, 0.8; F,58.1; S, 8.9. Found: C, 23.7; H, 1.0; F, 57.3; S, 9.1.

EXAMPLE 15

A tubular reactor made of stainless steel having a diameter of 3 cm anda length of 30 cm is filled with 100 cc of Na₂ CO₃. While passing drynitrogen through the reactor at the rate of 250 cc/min., the filler bedis heated externally by means of an electric heater at 350° C. to bepreliminarily dried. After preliminary drying is continued for 4 hours,the rate of dry nitrogen passed is changed to 50 cc/min. and, whilemaintaining the filler bed at 185° to 190° C., the compound ##STR63##(120 g) as prepared in Example 11 is fed into the tubular reactor at therate of 30 g/hr. The vapor emitted from the bottom of the tube iscondensed and collected in a trap cooled by dry ice-methanol. The liquidcomposition is distilled to obtain 70 g of the fraction of distillate at77°-80° C./100 mm Hg.

Said fraction is identified by elemental analysis, IR and NMR spectra tohave the structure of H₅ C₂ SCF₂ CF₂ CF₂ OCF═CF₂

IR (liquid): 2960, 2930, 2870 cm⁻¹ (C₂ H₅ --), 1840 cm⁻¹ (CF₂ ═CFO--),1100°-1300 cm⁻¹ (--CF₂ --).

Elemental analysis: C₇ H₅ F₉ OS: Calculated: C, 27.3; H, 1.6; F, 55.5;S, 10.4. Found: C, 27.1; H, 1.8; F, 55.0; S, 10.3.

EXAMPLE 16

Example 15 is repeated except that the compound ##STR64## (120 g)prepared in Example 13 is used in place of the compound ##STR65## As theresult, distillation of the reaction product gives 50 g of the fractionof distillate boiling at 82°-86° C./10 mm Hg. Said fraction isidentified by elemental analysis, IR and NMR spectra to have thestructure of H₅ C₂ O₂ SCF₂ CF₂ CF₂ OCF═CF₂.

IR (liquid): 2960, 2930, 2870 cm⁻¹ (C₂ H₅ --), 1840 cm⁻¹ (CF₂ ═CFO--),1360 cm⁻¹ (SO₂), 1100-1300 cm⁻¹ (--CF₂ --).

Elemental analysis: C₇ H₅ F₉ O₃ S. Calculated: C, 24.7; H, 1.5; F, 50.3;S, 9.4. Found: C, 25.1; H, 1.7; F, 49.3; S, 9.6.

EXAMPLE 17

The compound ##STR66## as prepared in Example 11 is subjected tohydrolysis with an excess of an aqueous NaOH solution, followed bydehydration. The solid residue is washed several times with acetone toeffect extraction of the sodium carboxylate. The extract is evaporatedby an evaporator to remove acetone. The solid product is crushed andthoroughly dried under reduced pressure at 100° C. to obtain ##STR67## Around-bottomed glass flask of 500 cc capacity is equipped with astirrer, a heater and an outlet for effluent gas which is connected viaa trap cooled by dry ice-methanol to a vacuum line. In said flask, thereis charged 100 g of ##STR68## While maintaining the inner pressure at 10mm Hg under stirring, said compound is thermally decomposed at 200° C.for 2 hours. The condensed liquid in the trap is subjected to precisiondistillation to obtain 18 g of the fraction of distillate at 77°-80°C./100 mm Hg. Said fraction is identified by elemental analysis, IR andNMR spectra to have the structure of H₅ C₂ SCF₂ CF₂ CF₂ OCF═CF₂.

EXAMPLE 18

Example 15 is repeated except that the compound ##STR69## as prepared inExample 14 is used in place of ##STR70## As the result, there isobtained 65 g of the fraction of distillate boiling at 81° C./200 mm Hg.

Said fraction is identified by elemental analysis, IR and NMR spectra tohave the structure of CH₃ SCF₂ CF₂ CF₂ OCF═CF₂.

IR characteristic absorption (liquid): 3025, 2970, 2850 cm⁻¹ (--CH₃),1840 cm⁻¹ (CF₂ ═CFO--), 1300-1100 cm⁻¹ (--CF₂ --).

Elemental analysis: C₆ H₃ F₉ OS: Calculated: C, 24.5; H, 1.0; F, 58.2;S, 10.9. Found: C, 24.2; H, 1.2; F, 57.5; S, 11.1.

COMPARATIVE EXAMPLE 1

The procedure of Example 15 is repeated except that ##STR71## is usedand passed through the sodium carbonate bed in place of ##STR72##whereby no objective CF₂ ═CFO(CF₂)₂ SO₂ F is obtained but only thecyclized product ##STR73## can be obtained.

EXAMPLE 19 In a stainless steel autoclave of 300 cc capacity, there arecharged 10 g of CF₂ ═CFO(CF₂)₃ SC₂ H₅, 0.1 g of ammonium persulfate andwater. The mixture is emulsified using ammonium perfluorooctanoate asemulsifier and polymerized at 50° C. under the pressure of 15 kg/cm² oftetrafluoroethylene, while adding sodium hydrogen sulfite asco-catalyst, to prepare the copolymer of the present invention. As theresult of elemental analysis, this copolymer is found to contain 4.23%of sulfur.

This copolymer is formed into a thin film for measurement of attenuatedtotal reflection (ATR). As the result of measurement, there are foundabsorptions at 2980 cm⁻¹ due to ethyl group, 990 cm⁻¹ due to ether groupand 740 cm⁻¹ due to C--S--C.

The above copolymer is found to have a melt index of 1.6 g/10 min., asmeasured under the conditions of the temperature of 250° C. and the loadof 2.16 kg by means of a device with an orifice of 2.1 mm in diameterand 8 mm in length.

This copolymer is formed into a film with a thickness of 250μ andtreated with chlorine gas at 120° C. for 20 hours, followed further bytreatment with a saturated aqueous chlorine water at 83° C. for 20hours. The resultant film is subjected to measurement of ATR, wherebythe absorption by ethyl groups at around 3000 cm⁻¹ is found to bevanished and instead thereof there appears absorption due to sulfonylchloride groups at around 1420 cm⁻¹. The ion-exchange capacity ismeasured after hydrolyzing a part of said film with an alkali to be 1.3meq/g-dry resin, indicating that the ratio of the recurring units, i.e.##STR74##

One surface of this film having sulfonyl chloride groups is treated witha mixture comprising 57% hydroiodic acid and glacial acetic acid at avolume ratio of 15:1 at 72° C. for 18 hours and then hydrolyzed with analkali. Furthermore, the thus treated membrane is treated with anaqueous 5% sodium hypochlorite solution at 90° C. for 16 hours to obtaina cation exchange membrane. Measurement of the ATR of this membranegives the result that there are observed absorptions at 1690 cm⁻¹ due tocarboxylic acid salt form and at 1060 cm⁻¹ due to sulfonic acid saltform. When the cross-section of the membrane is stained with an aqueousMalachite Green solution adjusted at pH=2, the membrane is stained inblue to the depth of 12μ from the treated surface, the residual portionbeing stained in yellow. The gradient of carboxylic acid density in thelayer stained in blue is measured according to the following method.

According to the method similar to that described above, there isprepared a membrane having the same exchange capacity wherein all theion-exchange groups are converted to carboxylic acid groups. The ATR ofthis membrane is measured and absorbance of carboxylic acid salt at 1690cm⁻¹ is calculated according to the base line method, said absorbancebeing determined as 100. The surface layer on the side having carboxylicacid salt groups of the aforesaid membrane is scraped evenly and thescraped surface is subjected to measurement of ATR, from which theabsorbance of the carboxylic acid salt is calculated. The percentage A %is calculated based on the absorbance of the film of the above membranecontaining only carboxylic acid groups. On the other hand, thethicknesses before and after scraping are measured to determine thedifference Bμ therebetween. Thus, the density of carboxylic acid groupsin the thickneSS of Bμ from the surface layer is determined as A %.

The densities of carboxylic acid groups in the membrane of this Exampleas found in the scraped sections are 100% on the surface, 88% at thedepth of 5μ from the surface, 68% at the depth of 10μ, 46% at the depthof 15μ, 26% at the depth of 20μ and 0% at the depth of 29μ. Theaccompanying drawing shows the relation between the depth and thedensity, indicating the maximum density gradient of 4.4%/μ.

The electrolysis performance of said membrane is measured according tothe following method.

There is used an electrolytic cell comprising the anode compartment andthe cathode compartment separated by said membrane with a currentpassage area of 0.06 dm² (2 cm×3 cm) and said membrane is assembled inthe cell so that the surface having carboxylic acid groups may facetoward the cathode side. As the anode, a dimensionally stable metalelectrode is used and as the cathode an iron plate. Into the anodecompartment is charged a saturated aqueous sodium chloride solution andthe pH of the anolyte is maintained at 3 by the addition of hydrochloricacid. While 10 N aqueous caustic soda solution is circulated to thecathode compartment, water is added thereto in order to maintain theconcentration at a constant value.

While maintaining the temperatures in both the anode compartment and thecathode compartment at 95° C., current is passed at the current densityof 110 A/dm². The current efficiency is calculated by dividing theamount of caustic soda formed in the cathode compartment by thetheoretical amount calculated from the quantity of current passed.

The current efficiency and the cell voltage are measured with lapse oftime to obtain the following results:

    ______________________________________                                        Current passage time (hrs.):                                                                       24      720                                              Current efficiency (%):                                                                            93       93                                              Voltage (V):           4.7      4.7                                           ______________________________________                                    

After passage of current, the membrane is observed to have no physicaldamage such as water bubbles, cracks or peel-off.

COMPARATIVE EXAMPLE 2

In a stainless steel autoclave of 300 cc capacity, there are charged 10g of ##STR75## 0.1 g of ammonium persulfate and water. The mixture isemulsified using ammonium perfluorooctanoate as emulsifier andpolymerized at 50° C. under the pressure of tetrafluoroethylene of 3kg/cm², while adding sodium hydrogen sulfite as co-catalyst. Theion-exchange capacity of the resultant copolymer is measured afterhydrolysis of a part thereof to be 1.3 meq/g-dry resin. The ratio of therecurring units of this polymer, i.e. ##STR76## is found to be 3.3.

After washing the above polymer with water, the polymer is formed into afilm with a thickness of 250μ, which is in turn hydrolyzed with analkali. The resultant membrane is too low in mechanical strength toperform an evaluation thereof.

COMPARATIVE EXAMPLE 3

Comparative example 2 is repeated except that the pressure oftetrafluoroethylene is changed to 5 kg/cm². The resultant polymer isfound to have an ion-exchange capacity of 0.89 meq/g-dry resin. Saidpolymer is found to have a ratio of the recurring units, namely##STR77## of 6.8.

After the above polymer is washed with water, it is molded into a filmwith a thickness of 250μ and then hydrolyzed with an alkali. This filmis thoroughly dried and then treated at 110° C. for 20 hours byimmersing said film in a mixture comprising phosphorus pentachloride andphosphorus oxychloride at a weight ratio of 1:3. By measurement of theATR of this membrane, there appears specific absorption at 1420 cm⁻¹ dueto sulfonyl chloride groups. After treatment of one surface of saidmembrane with 57% hydroiodic acid at 83° C. for 20 hours, the treatedsurface is hydrolyzed with an alkali, followed further by treatment withan aqueous 5% sodium hypochlorite solution at 90° C. for 16 hours. Bymeasurement of the ATR of the membrane, there is observed specificabsorption at 1690 cm⁻¹ on the treated surface due to carboxylic acidsalt. When the cross-section of the membrane is stained similarly as inExample 19, the membrane is found to be stained in blue to the depth of8.6μ from the surface, the residual portion being stained in yellow.

This membrane is provided for electrolysis evaluation according to thesame method as described in Example 19, with the surface havingcarboxylic acid groups facing toward the cathode side. The currentefficiency and the voltage are measured to give the following results:

    ______________________________________                                        Current passage time (hrs.):                                                                       24      720                                              Current efficiency (%):                                                                            94       86                                              Voltage (V):           5.6      6.1                                           ______________________________________                                    

After passage of current, the membrane surface subjected to currentpassage is observed to find that there are water bubbles. Thecross-section of the membrane is also observed to find that there ispeel-off in the carboxylic acid layer at the depth of 5μ from thesurface layer.

COMPARATIVE EXAMPLE 4

Polymerization is conducted in the same manner as in Comparative example2 except that the pressure of tetrafluoroethylene is changed to 5kg/cm². A part of the resultant polymer is hydrolyzed to give anion-exchange resin having an ion-exchange capacity of 0.83 meq/g-dryresin. This polymer is molded into a film with a thickness of 50μ. Thisfilm is called film a.

On the other hand, 16 g of CF₂ ═CFO(CF₂)₃ COOCH₃, 0.17 g of ammoniumpersulfate and water are charged into a stainless steel autoclave of 500cc capacity. The mixture is emulsified using ammonium perfluorooctanoateas emulsifier and polymerization is carried out at 50° C. under thepressure of tetrafluoroethylene of 7 kg/cm² using sodium hydrogensulfite as co-catalyst. A part of the polymer is subjected to hydrolysisand the hydrolyzed product is found to have an ion-exchange capacity of1.10 meq/g-dry resin. This polymer is molded into a film with athickness of 100μ. This film is called film b.

The film a is placed on the film b and the resultant composite issubjected to press molding to give a laminated membrane. This membrane,after hydrolysis with an alkali, is evaluated for its electrolysisperformance with the surface of the film b facing toward the cathodeside. The results are shown below:

    ______________________________________                                        Current passage time (hrs.):                                                                        24    720                                               Current efficiency (%):                                                                             92    84                                                Voltage (V):          18    20                                                ______________________________________                                    

After passage of current, the membrane subjected to the passage ofcurrent is found to have water bubbles formed on the entire surface. Byobservation of the cross-section of the membrane, there is foundpeel-off at exactly the interface between the film a and the film b.

COMPARATIVE EXAMPLE 5

Example 19 is repeated except that ##STR78## and CF₂ ═CFO(CF₂)₄ COOCH₃are used in place of CF₂ ═CFOCF₂ CF₂ CF₂ SC₂ H₅ and copolymerization iscarried out while blowing tetrafluoroethylene according to the method asdescribed in Example 2 of Japanese published unexamined patentapplication No. 120492/1975. This polymer is molded into a film with athickness of 250μ and, after hydrolysis with an alkali, evaluated forits electrolysis performance according to the method as described inExample 19. The results are shown below.

    ______________________________________                                        Current passage time (hrs.):                                                                       24      720                                              Current efficiency (%):                                                                            89       82                                              Voltage (V):           4.5      4.4                                           ______________________________________                                    

COMPARATIVE EXAMPLE 6

One surface of the sulfonyl chloride type membrane obtained inComparative example 3 is treated with a perfluoro-dimethylcyclobutanesolution containing 5 wt. % of CF₂ ═CFO(CF₂)₃ COOCH₃ and a catalyticamount of azobisisobutyronitrile at 50°-60° C. for 5 hours. After saidtreatment, the membrane is subjected to hydrolysis treatment with 2.5Ncaustic soda/50% aqueous methanol solution. As the result of measurementof the ATR of the treated surface, there is found specific absorption ofcarboxylic acid salt at 1690 cm⁻¹. When the cross-section of themembrane is stained with Malachite Green, the layer with the thicknessof 4μ from the treated surface is found to be stained in blue.

Evaluation of electrolysis performance of the membrane is performedsimilarly as in Example 19, with the surface having carboxylic acidgroups facing toward the cathode side, to give the following results.

    ______________________________________                                        Current passage time (hrs.):                                                                       24      720                                              Current efficiency (%):                                                                            91       80                                              Voltage (V):           5.7      6.2                                           ______________________________________                                    

The membrane surface subjected to current passage is found to have waterbubbles formed on its entire surface.

COMPARATIVE EXAMPLE 7

According to the method similar to Comparative example 4, there areprepared a copolymer a of tetrafluoroethylene with ##STR79##(ion-exchange capacity after hydrolysis=0.91 meq/g-dry resin) and acopolymer b of tetrafluoroethylene with ##STR80## (ion-exchange capacityafter hydrolysis=0.92 meq/g-dry resin). The copolymers a and b areblended at a weight ratio of 50/50 on a roll mill and then press moldedinto a film of 100μ in thickness. This film is called film A.Separately, the copolymer a is press molded into a film of 100μ inthickness. This film is called film B. The films A and B are placed oneach other and press molded into a laminated membrane. This membrane issubjected to hydrolysis with an alkali and thereafter its electrolysisperformance is measured similarly as in Example 19, with the surface ofthe film A facing toward the cathode side. The results are shown below.

    ______________________________________                                        Current passage time (hrs.):                                                                       24      720                                              Current efficiency (%):                                                                            90       82                                              Voltage (V):           7.3      9.0                                           ______________________________________                                    

After passage of current, water bubbles are found to be formed all overthe area subjected to current passage. The cross-section of the membraneis observed to find that there occurs peel-off exactly at the interfacebetween the laminated films A and B.

EXAMPLES 20-21

Example 19 is repeated except that the reducing agents and the treatmentconditions for treatment of one surface having sulfonyl chloride groupsare changed as shown in Table 1. The elctrolysis performance, thesurface density and the maximum density gradient of carboxylic acidgroups are also shown in Table 1.

In any of the membranes after passage of current, there is observed nowater bubbles, peel-off or cracks.

                                      TABLE 1                                     __________________________________________________________________________                        Electrolysis                                                                           Surface                                                                              Maximum                                                       performance*                                                                           density                                                                              density                                   Example                                                                            Reducing agent and treatment                                                                 After                                                                             After                                                                              of     gradient                                  No.  conditions     24 hrs.                                                                           720 hrs.                                                                           --COOH (%)                                                                           (%/μ)                                  __________________________________________________________________________    20   57% hydroiodic acid-                                                                         95/4.8                                                                            95/4.8                                                                             100    4.5                                            propionic acid (15:1,                                                         volume ratio) mixture,                                                        72° C., 18 hours                                                  21   57% hydroiodic acid-                                                                         94/4.8                                                                            94/4.8                                                                             100    4.1                                            caprylic acid                                                                 (500:1, weight ratio)                                                         mixture, 83° C., 20 hours                                         __________________________________________________________________________     *Electrolysis performance: Current efficiency (%)/Voltage (V)            

EXAMPLE 22

Polymerization is carried out in the same manner as in Example 19 exceptthat CF₂ ═CFO(CF₂)₃ SC₂ H₅ and ##STR81## are charged at a molar ratio of4:1. The resultant polymer is treated similarly as described in Example19. The results obtained are similar to those as reported in Example 19.

EXAMPLE 23

Polymerization is carried out in the same manner as in Example 19 exceptthat the pressure of tetrafluoroethylene is changed to 17 kg/cm². Theion-exchange capacity of a part of the resultant polymer is measured bythe same method as in Example 19 to be 0.75 meq/g-dry resin. The ratioof the recurring units in this polymer, i.e. ##STR82## is found to be10. The above polymer is molded into a film with a thickness of 50μ.This film is called film c. The sulfide type polymer obtained in Example19 is also molded into a film with a thickness of 100μ. This film iscalled film d. The film c is placed on the film d and the composite ispress molded into a laminated membrane. Then, with the side of the filmd downward, said membrane is placed on a fabric made ofpolytetrafluoroethylene, which is "leno-woven" fabric with a thicknessof about 0.15 mm comprising wefts of 400 denier multi-filaments andwarps of 200 denier multi-filaments×2. By heating the membrane undervacuo, the fabric is embedded in the film d to reinforce said membrane.

The laminated membrane incorporated with a reinforcing material issubjected to the chlorine treatment similarly as in Example 19 to form asulfonyl chloride type laminated membrane. Said laminated membrane istreated on the side of the film c with a mixture comprising 57%hydroiodic acid and glacial acetic acid at a volume ratio of 10:1 at 83°C. for 20 hours. After hydrolysis with an alkali, the membrane isfurther treated with 5% sodium hypochlorite at 90° C. for 16 hours. Whenthe cross-section of the resultant membrane is stained with an aqueousMalachite Green solution adjusted at pH 2, the layer with thickness of11μ from the surface of the film c is stained in blue, the residual partbeing stained in yellow. The maximum density gradient of carboxylic acidgroups in the layer stained blue is measured to be 4.9%/μ, and thedensity of carboxylic acid groups on the surface to be 92%.

When the electrolysis performance of the membrane is measured accordingto the same method as described in Example 19 using 6N alkaliconcentration, with the side of the film c facing toward the cathodeside, there are obtained the following results. The membrane subjectedto passage of current is free from water bubbles, peel-off or cracks.

    ______________________________________                                        Current passage time (hrs.):                                                                       24      720                                              Current efficiency (%):                                                                            93       93                                              Voltage (V):           5.5      5.5                                           ______________________________________                                    

EXAMPLES 24-27

The laminated membrane prepared in Example 23 is treated on the side ofthe film c with the reducing agents and under the treatment conditionsas shown in Table 2, followed by subsequent treatments similarlyconducted as in Example 23. The electrolysis performance, the density ofcarboxylic acid groups on the surface of the film c and the maximumdensity gradient are set forth in Table 2.

None of these membrane show water bubbles, peel-off or cracks after thepassage of current.

                                      TABLE 2                                     __________________________________________________________________________                       Electrolysis                                                                  performance*                                                                            Surface                                                             After 24                                                                           After 720                                                                          density of                                                                          Maximum                                                       hours'                                                                             hours'                                                                             carboxylic                                                                          density                                    Example                                                                            Reducing agent and                                                                          current                                                                            current                                                                            acid  gradient                                   No.  treatment conditions                                                                        passage                                                                            passage                                                                            groups (%)                                                                          (%/μ)                                   __________________________________________________________________________    24   57% hydroiodic acid-                                                                        92/5.4                                                                             92/5.4                                                                             84    4.0                                             glacial acetic acid (8:1,                                                     volume ratio) mixture,                                                        83° C., 15 hours                                                  25   47% hydrobromic acid-                                                                       91/5.4                                                                             91/5.4                                                                             68    2.9                                             glacial acetic acid (3:1,                                                     volume ratio) mixture,                                                        90° C., 16 hours                                                  26   30% hypophosphorous acid-                                                                   90/5.3                                                                             90/5.3                                                                             54    2.4                                             propionic acid (3:1,                                                          volume ratio) mixture,                                                        90° C., 16 hours                                                  27   57% hydroiodic acid-                                                                        92/5.4                                                                             92/5.4                                                                             81    4.2                                             perfluorooctanoic acid                                                        (500:1, weight ratio)                                                         mixture, 72° C., 16 hours                                         __________________________________________________________________________     *Electrolysis performance: Current efficiency (%)/Voltage (V)            

EXAMPLE 28

In a stainless steel autoclave of 500 cc capacity, there are charged1,1,2-trichloro-1,2,2-trifluoroethane and CF₂ ═CFO(CF₂)₃ SO₂ C₂ H₅ andperfluoropropionyl peroxide as initiator, and polymerization is carriedout at 45° C. under the pressure of tetrafluoroethylene of 15 kg/cm².The resultant polymer is found to contain 4.10% of sulfur, as measuredby elemental analysis.

A part of this polymer is hydrolyzed with an alkali containing potassiumpermanganate and the ion-exchange capacity of the hydrolyzed polymer ismeasured to be 1.31 meq/g-dry resin.

The above sulfonic type polymer is molded into a membrane with athickness of 250μ, which is then hydrolyzed with an alkali containingpotassium permanganate. Subsequently, said membrane is immersed in amixture comprising 1:3 (weight ratio) of phosphorus pentachloride andphosphorus oxychloride to be treated at 110° C. for 20 hours.Measurement of the ATR of the resultant membrane gives the result thatthere appears specific absorption of sulfonyl chloride groups at 1420cm⁻¹.

After one surface of said sulfonyl chloride type membrane is treatedwith a mixture comprising 15:1 (volume ratio) of hydroiodic acid andpropionic acid at 72° C. for 18 hours, the treated membrane is subjectedto hydrolysis treatment with an alkali, followed further by treatmentwith an aqueous 5% sodium hypochlorite solution at 90° C. for 16 hours.When the cross-section of the membrane is stained with an aqueousMalachite Green solution, the layer with a thickness of 11μ from onesurface is found to be stained in blue, while the remaining portion isstained in yellow. The surface density and the maximum density gradientof carboxylic acid groups in the layer stained in blue are found to be100% and 5.1%/μ, respectively.

EXAMPLE 29-32

One surface of the sulfonyl chloride type membrane prepared in Example28 is treated similarly as in Example 28 using various reducing agentsand treatment conditions as shown in Table 3. The density of carboxylicacid groups on the surface of the membrane and the maximum densitygradient of carboxylic acid groups are also shown for each membraneobtained in Table 3.

                  TABLE 3                                                         ______________________________________                                                                Surface   Maximum                                                             density   density                                                             of        gradient of                                                         carboxylic                                                                              carboxylic                                  Example                                                                              Reducing agent and                                                                             acid      acid                                        No.    treatment conditions                                                                           groups (%)                                                                              groups (%/μ)                             ______________________________________                                        29     47% hydrobromic  69        4.9                                                acid-caprylic acid                                                            (500:1, weight ratio)                                                         mixture, 90° C., 30 hrs.                                        30     30% hypophosphorous                                                                            60        3.0                                                acid-glacial acetic                                                           acid (5:1, volume                                                             ratio) mixture,                                                               90° C., 16 hrs.                                                 31     30% hypophosphorous                                                                            75        4.9                                                acid-perfluorooctanoic                                                        acid (500:1, weight                                                           ratio) mixture, 83° C.,                                                24 hrs.                                                                32     57% hydroiodic acid-                                                                           94        4.6                                                perfluoroheptane-                                                             sulfonic acid (550:1,                                                         weight ratio) mixture,                                                        90° C., 16 hrs.                                                 ______________________________________                                    

EXAMPLE 33

The compolymer prepared according to the polymerization method asdescribed in Example 19 is extruded into a strand, which is then cutinto a granular resin of 1 mm in size by means of a pelletizer.

The functional groups containd in said resin are converted to sulfonylchloride groups by the method as described in Example 19, followed byhydrolysis to be converted to sulfonic acid groups. Then, theion-exchange capacity of the resin is measured to be 1.3 meq/g-dryresin.

EXAMPLE 34

An emulsion is formed in a stainless steel autoclave of 300 cc capacityby charging 10 g of CF₂ ═CFO(CF₂)₃ SCH₃, 1.0 g of sodium hydrogenphosphate, 45 cc of purified water and 0.45 g of ammoniumperfluorooctanoate. Then, 5 cc of 0.62% aqueous ammonium persulfatesolution is added to the emulsion and polymerization is carried out,while maintaining the temperature at 40° C., under the pressure oftetrafluoroethylene of 13 kg/cm², whereby the pressure oftetrafluoroethylene is controlled so as to keep constant thepolymerization rate. The resultant polymer is found to contain 3.50 wt.% of sulfur by elemental analysis. This polymer is press molded into athin film at 280° C. and subjected to measurement of ATR, whereby thereis observed absorption of methyl groups at 3000 cm⁻¹.

The above polymer is molded into a membrane with a thickness of 150μ,which is in turn treated with chlorine gas at 120° C. for 20 hours.Measurement of the ATR of the membrane gives the result that theabsorption of methyl groups at around 3000 cm⁻¹ has vanished.Furthermore, said membrane is treated with a liquid saturated withchlorine, comprising a mixture of perfluorobutyric acid and water (2:1,volume ratio) having dissolved chlorine therein, at 100° C. for 48hours. Measurement of the ATR of said membrane shows that there appearsabsorption of sulfonyl chloride groups at around 1420 cm⁻¹. Ion-exchangecapacity of said membrane is determined after hydrolysis of a partthereof with an alkali to be 1.04 meq/g-dry resin. The ratio of therecurring units of the membrane, i.e. ##STR83## is found to be 6.7.

One surface of the above sulfonyl chloride type membrane is treated witha mixture comprising 57% hydroiodic acid and acetic acid at 30:1 (volumeratio) at 72° C. for 16 hours, followed by hydrolysis with an alkali,and further treated with an aqueous 5% sodium hypochlorite solution at90° C. for 16 hours. By staining the cross-section of one surface of themembrane, the layer on one side of the membrane with a thickness of 12μis found, to be stained in blue, while the remaining portion is stainedin yellow. Electrolysis performance is measured under the sameconditions as used in Example 19, with the surface stained in blue ofthe membrane facing toward the cathode side, to give the result as shownbelow. The density of carboxylic acid groups and the maximum densitygradient are also measured to give the following values.

    ______________________________________                                        Electrolysis performance                                                      After    After        Surface   Maximum                                       24 hours'                                                                              720 hours'   carboxylic                                                                              density                                       current  current      acid group                                                                              gradient                                      passage  passage      density (%)                                                                             (%/μ)                                      ______________________________________                                        95%/5.0  95%/5.0      100       4.2                                           ______________________________________                                    

EXAMPLE 35

An emulsion is formed in a stainless steel autoclave of 300 cc capacityby charging 10 g of CF₂ ═CFO(CF₂)₃ SC₂ H₅, 0.1 g of ammonium persulfateand water, using ammonium perfluorooctanoate as emulsifier.Tetrafluoroethylene is pressurized into the autoclave at 15 kg/cm² andpolymerization is carried out at 50° C. by adding sodium hydrogensulfite as co-catalyst. The resultant polymer is found to contain 4.23wt. % of sulfur by elemental analysis. This polymer is molded into amembrane with a thickness of 250μ, which is in turn treated withchlorine gas at 120° C. for 20 hours, followed further by treatment witha saturated aqueous chlorine solution at 83° C. for 20 hours. The ATR ofthis membrane is measured, whereby the absorption appearing at around3000 cm⁻¹ before chlorine treatment is found to have vanished andinstead there appears absorption of sulfonyl chloride groups at around1420 cm⁻¹. Ion-exchange capacity of said membrane is measured afterhydrolysis with an alkali to be 1.3 meq/g-dry resin, indicating that theratio of the recurring units, i.e. ##STR84##

The electrolysis performance of said membrane is measured according tothe following method.

There is used an electrolytic cell comprising the anode compartment andthe cathode compartment separated by said membrane with a currentpassage area of 0.06 dm² (2 cm×3 cm) and said membrane is assembled inthe cell. As the anode, a dimensionally stable electrode is used and asthe cathode an iron plate. A saturated aqueous sodium chloride solutionis charged flown into the anode compartment and adjusted at pH 3 byadding hydrochloric acid thereto. While 13N aqueous caustic sodasolution is circulated to the cathode compartment, water is addedthereto in order to maintain the concentration at a constant value.

While maintaining the temperatures in both the anode compartment and thecathode compartment at 110° C., current is passed at the current densityof 120 A/dm². The current efficiency is calculated by dividing theamount of caustic soda formed in the cathode compartment by thetheoretical amount calculated from the quantity of current passed to be65%. After current is passed for 700 hours, there is observed nophysical damage on the membrane such as bubble formation, cracks orpeel-off.

EXAMPLE 36

An emulsion is formed by charging 10 g of CF₂ ═CFO(CF₂)₃ SCH₃, 1.0 g ofsodium hydrogen phosphate, 45 cc of purified water and 0.45 g ofammonium perfluorooctanoate in a stainless steel autoclave of 300 cccapacity. Then, 5 cc of an aqueous 0.62% ammonium persulfate solution isadded to the mixture, and polymerization is conducted under the pressureof tetrafluoroethylene of 13 kg/cm², while maintaining the temperatureat 40° C. During the polymerization, the pressure of tetrafluoroethyleneis controlled so as to keep constant the rate of polymerization. Theresultant polymer is found to contain 3.5 wt. % of sulfur by elementalanlaysis. This polymer is press molded at 280° C. into a thin film,which is subjected to measurement of ATR, whereby it is found that thereis observed absorption of methyl groups at around 3000 cm⁻¹.

A membrane with a thickness of 150μ prepared by molding of the abovepolymer is treated with chlorine gas at 120° C. for 20 hours, wherebyabsorption of methyl groups at around 3000 cm⁻¹ is found to havevanished as measured by the ATR of the membrane. Further, said membraneis treated with a liquid saturated with chlorine, comprising a mixtureof perfluorobutyric acid and water at 2:1 (volume ratio) havingdissolved chlorine therein, at 100° C. for 48 hours. Measurement of theATR of the resultant membrane gives the result that there appearsabsorption of sulfonyl groups at around 1420 cm⁻¹. The ion-exchangecapacity of said membrane is measured after hydrolysis of said membranewith an alkali to be 1.04 meq/g-dry resin, thus giving the ratio of therecurring units of the membrane, i.e. ##STR85## of 6.7.

EXAMPLE 37

The polymer prepared in Example 19 is molded into a film with athickness of 200μ. A fabric made of polytetrafluoroethylene fibers isembedded in this film according to the following method. The device usedin this embedding procedure comprises two aluminum plates, each beingprovided on the upper surface by mechanical working with a series ofgrooves so as to create a pressure difference across the upper surfaceof the plate. The pressure difference is applied through the hole boredthrough the side surface of the plate, said hole being connected to thegrooves on the upper surface of the plate. On this plate is placed a60-mesh wire-screen so that the pressure difference may be applied onevery point on the upper surface. A sheet of asbestos paper is placed onthe upper surface of the wirescreen, screen and on said sheet issuperposed a "leon-woven" fabric with a thickness of about 0.15 mm madeof polytetrafluoroethylene fibers comprising, each 25 per inch, 400denier multi-filaments as weft and 200 denier multifilaments×2 as warp.On said fabric is further placed the above film. The size of the film ismade slightly larger than the other components and the marginals of thesheets of the fluorinated polymer are fastened onto the aluminum plateswith a tape, thus forming an air-tight package.

The embedding device is placed between the electrically heated hotplates, whereby the hot plate contacted with the aluminum plate ismaintained at 300° C. and the hot plate contacted with the film at 180°C. for 5 minutes. Then, through the hole on the side surface of thealuminum plate, evacuation is effected to provide a 100 mm Hg pressuredifference. Under such conditions, the whole composite is left to standfor 3 minutes. The temperature of the hot plates is then cooled to roomtemperature and the pressure difference is removed. By observation ofthe cross-section of the film, the fabric is completely embedded withinthe film.

When the thus prepared membrane is treated with chlorine gas andsubsequent treatments as described in Example 19, there is obtained amembrane having similar current efficiency according to the sameevaluation test of electrolysis performance as described therein.

We claim:
 1. A fluorinated carboxylic acid derivative having theformula:

    X(CF.sub.2).sub.2 Y

wherein X is --SR¹ or --SO₂ R² ; R¹ is C₁ -C₁₀ alkyl, C₁ -C₁₀perfluoroalkyl or chlorine; R² is R¹ or --OM where M is hydrogen, onalkali metal or ammonium; Y is --COY¹ or --CN; Y¹ is halogen, hydrogen,--NH₂, --OM wherein M is defined above, or --OR³ ; and R³ is C₁ -C₁₀alkyl.
 2. A fluorinated carboxylic acid derivative according to claim 1,wherein R¹ and R² are --C₂ H₅, --CH₃ or Cl.
 3. A fluorinated carboxylicacid derivative according to claim 1, wherein Y is --COY¹.
 4. Afluorinated carboxylic acid derivative according to claim 3, wherein Y¹is --F, --Cl, --OH, --ONa or --OCH₃.